Spoiler:
Just replayed game, I can get a general size of floating continent and applicable feats from some context clues without relying on assuming the world is Earth sized.

Namely Cid travels with you over the course of 2 days from Kazus to Canaan.

I can say this without doubt, given the context of the two he owes you stems from taking him back to Canaan in the first place and proceeding to heal his Wife's illness with an elixir when you get there.

The first half of the trip?

By airship. You lose the airship upon it being totaled in a massive explosion upon colliding with a mountain sized boulder.

Second half is all walking.

So, how am I going to do this?

Well, the airship has no official speed, but I figure using one of the lower end cruising speeds I found of about 90 mph (40.294 m/s) for a helicopter would fit the bill (these things run off antimatter for reference so... I might be understating how little of the two days they were in the air here)

The walking speed? 3 mph (1.341 m/s) is average IIRC.

I looked up the source after I did the math... so everyone can fuck off about me not using exactly what's in the source

So, on with the calc.

Spoiler:

By Flying = 257 pixels

By Walking = 258 pixels

Alright, cool. Just have them both for sake of comparison. I'm using walking though for the actual figure.

Bahamut Flight = 166 pixels (he has two such feats, this one is the one I'm measuring though)

Continent Diameter = 1856 pixels

Boulder Diameter = 16 pixels

Castle Hein Diameter = 25 pixels

Castle Hein Height = 26 pixels

Height Above Ground = 6 pixels

Now... how long am I going to suggest over a period of 2 days that they traveled?

Well, let's say they took 8 hours to sleep both days, so subtract 16 hours from the original 48. Also account for any eating or bathroom breaks, maybe chop off another 4 hours a day?

I'm more than willing to discuss what would be considered reasonable here... given I'm taking a shot in the dark saying 12 hours of travel of day given Cid's urgency would make sense to me.

Which comes out to 24 hours of travel total FYI.

Or better known as...

Total Time Frame = 86,400 seconds

Airship Speed = 40.294 m/s

Average Walking Speed = 1.341 m/s

Now to determine how long they were in the airship compared to walking.

By Walking/By Airship = 0.033

Airship Time Frame = 2,851.2 seconds

Which you then subtract from the total time to get the walking time of...

Walking Time Frame = 83,548.8 seconds

And at an average walking pace you get...

Walking Distance = 112,038.941 meters

Bahamut Flight/By Walking = 0.643

Continent Diameter/By Walking = 7.194

Boulder Diameter/By Walking = 0.062

Castle Hein Diameter/By Walking = 0.097

Castle Hein Height/By Walking = 0.101

Height Above Ground/By Walking = 0.023

Bahamut Flight = 72,041 meters

Time Frame = 1 second (10 frames at 10 fps)

Bahamut Flight Speed = 72,041 m/s or mach 211.705

Continent Diameter = 806,008.142 meters

Boulder Diameter = 6,946.414 meters

Before anyone thinks to bitch, the boulder was blocking a path through an entire valley... which we get to see a from the ground shot of here.

Looks like a fuckton of distance between Luneth's position and the nearest mountain, so the boulder on the map is likely portrayed to scale.

Volume = (4/3)PIr^3

r = Boulder Diameter/2

Boulder Volume = 175,501,410,900 m^3

214.35 j/cc (it was pretty clearly pulverized if you bothered watching the video I linked)

Airship Ramming Energy/Beginning of Game Warriors of
Light Durability = 37,618,727,430,000,000,000 joules or 8.991 gigatons

This is the first of a running gag in the game FYI... you always seem to lose your fucking airship to a crash.

Now onto Hein, who enchanted the Elder Tree into floating and acting as a mobile fortress. I have nothing to scale it from but the world map, and nothing particularly contradicts the scale (barring the Living Woods replanting scene... where it looks too small to have ever housed a dungeon [It looks no more than about 5 meters thick there] so we kind of just have to write that scene off)

Castle Hein Diameter = 10,867.777 meters

Castle Hein Height = 11,315.933 meters

Height Above Ground = 2,576.896 meters

Volume = 0.2((4/3)PIr1^2r2)

r1 = Castle Hein Diameter/2

r2 = Castle Hein Height/2

Castle Hein Volume = 139,958,868,300 m^3

Wood Density = 600 kg/m^3 (a fairly average wood density IIRC)

Castle Hein Mass = 839,753,209,800,000 kilograms

PE = mgh

m = Castle Hein Mass

g = 9.8 m/s^2

h = Height Above Ground

PE of Castle Hein = 2,120,677,554,000,000,000 joules or 506.854 megatons

Now for the floating continent.

Why?

It was sent into the sky as a byproduct of the Darkness Xande spread across the planet to halt time.

Spoiler:

Floating Continent Diameter = 35 pixels

Floating Continent Height = 36 pixels

Elevation = 57 pixels

Floating Continent Height/Floating Continent Diameter = 1.029

Elevation/Floating Continent Diameter = 1.629

Floating Continent Height = 829,382.378 meters

Elevation = 1,312,987.263 meters

Volume = (2/3)PIr1^2r2

r2 = continent height

r1 = continent diameter/2

Floating Continent Volume = 282,118,966,200,000,000 m^3

Continental Crust Density = 2,700 kg/m^3

Floating Continent Mass = 761,721,208,700,000,000,000 kilograms

PE = mgh

I'd normally use one to account for change in gravity as you drastically increase in altitude as seen here... except for the fact no one ever notes the surface world possesses greater gravity than on the Floating Continent... nor does it have shit like fal'Cie to induce artificial gravity like FFXIII could in explanation of similar gravity between Pulse and Cocoon.

Potential Energy = 9,801,276,401,000,000,000,000,000,000 joules or 2.343 exatons

Notably? After being launched into the sky, to keep it stable, the Tower of Owen was built. The tower was based on technology that could move planets according to the fucker that invented it. For over 1000 years it did its job without issue until Medusa fucked with the reactor. Not only does Desch jump into the heart of all the energy keeping the fucking thing in check and live (because exaton level durability is cool I guess for an Ancient), he also implies without the tower and reactor, the floating continent would be thrown from the "sun"

Which I can actually confirm in the raw even suggests the Floating Continent will be thrown from the Solar System.

Japanese text with sun word boxed.

Kanji or whatever the fuck for sun is: "太陽"

The rest translated here: "このままでは浮遊大陸は太陽からはなれてしまいどうなるかわからない
The way things are going, the Floating Continent might even drift away from the sun..."

So that means the energy being used to keep the Floating Continent near the planet, and what was used to launch it into the air initially, was much higher than just PE.

Escape Velocity from Sun Relative to Earth's position in Solar System = 42,100 m/s (according to the chart here on wikipedia anyway)

KE = 675,041,143,755,983,500,000,000,000,000 joules or 161.339 exatons

Final Tally

Bahamut Flight Speed = 72,041 m/s

Airship Ramming Energy/Post-Wind Crystal Minimum Warriors of Light Durability = 8.991 gigatons

Hein's Magical Power = 506.854 megatons

Energy Required to Send Floating Continent into the Sky (PE Derived) = 2.343 exatons

Energy Required to Send Floating Continent into the Sky (KE Derived) = 161.339 exatons