Above is a photo of the Xenoblade X map, with NLA overlayed to approximate through exhaustion the number of times the area XCX's world is in comparison to NLA. It is about 21 NLA's in horizontal space. This is of course ignoring the vertical dimension and says nothing about surface area. 

Now below is the size of NLA. 

Notice the size of the houses in the top right portion of NLA. There are twelvish city blocks in that area of varying sizes. But in the real world, the average size of a city block is about 100,000 sq ft, and that seems to work out similary to what we see, more or less based on the number of houses and the scale. So 12 blocks * 100,000 average sq ft/block = 1,200,000 sq ft.

So now there are two parts I want to estimate. The area of the inner perimeter and the flat area of the barrier. 

Both of these will be very rough estimates. It looks to me as if the grassy area above is about 1/10th of the inner perimeter's area, based on the curve length and intuition mostly. That would make the inner portion of NLA about 12,000,000 SqFt or almost half a square mile. The radius from the center to the outer portion (where the crystals end) looks to me to be twice that of the radius from the center to the inner wall. So using our estimate of 12,000,000 SqFt, we can find the radius of the inner portion to the inner wall to be approximately 1954 ft, and then multiply that by two, and use our formula for the area of a circle again to find the total area to be approximately 48,000,000 sqft (makes sense because twice the radius implies four times the area for a circle), or about 1.7 sqmi. Now using our estimate that the land, if it were flat, is about 21 NLA's we get 1.72 sqmi * 21 = 36.21 sqmi or 93.57 square kilometers. 

Now of course we are not including vertical land such as a hill, mountain, or pillar and also ocean size into this estimate. Takahashi said 400 km^2. If I filled the whole map with NLA I'd get something like 100 NLA's (I found NLA to be about a square of 5*5 on a grid in which the map is 50*50), which would give 440 km^2. So either NLA is much bigger than I estimate it to be (about four times bigger, which would imply a two mile radius and a four mile diameter, way too big for the pictures we've seen), or Takahashi is including ocean size in his estimate. 

Overall I expect the actual land surface area to come out to be somewhere between 100 km^2 - 300 km^2 and the total surface area including the oceans to be over 400 km^2. This also goes with the more than  5 times larger than Xenoblade, because Xenoblade's surface area was about 29 km^2. 

As for the area of each continent (not considering surface area) 

Grass : 26 km^2

Desert: 17 km^2

Forest: 17 km^2 (added one NLA for missing space) 

Ash: 22 km^2 (added one NLA for missing space)

Volcano: 17 km^2 

Each continent should be roughly the same size as the entire land area of the original Xenoblade. It's gonna be nuts.

Yep, it seems like that from everything we've been told and the relative size of NLA to the rest of the world.  The grassland continent might even be bigger than Xenoblade, by itself. 

Not a bad analysis, actually. Very impressive! And I agree with your ballpark figures, too.

Thanks! I treated this like a fermi problem with the technique of the method of exhaustion. 

It was actually fun to do, and I thought I wouldn't want to do any math/physics during spring break, haha.