So here is a seldom recognized factor of building a building.

You suggested measurements of 48' x 16' = 768 Sq. Ft.

That is 48' + 48'+ 16' + 16' = 128 linear feet of exterior wall

Or, with the same linear feet of wall, 128' x 4 = 32 LF per side

Then 32' x 32' = 1,024 Sq. Ft.

The point being, the closer you get to building a square vs. a long and skinny rectangle, the more usable area or interior volume you get for the same amount of wall materials.

The only reason for building a 48' x 16' is if you wanted to make 3 pens of 16' x 16' or 6 pens of 8' x 16'. In any case, you can safely use the factor of 4 SF of floor space per bird as a starting point. So your 768 SF building should support:

768 / 4 = 192 birds

Build it square at or perhaps 28' x 36' (also 128 LF) and you get 1,008 SF

Then 1,008 SF / 4 = 252 birds

or 24' x 40' (making two pens of 24' x 20' or 480 SF / 4 = 120 birds

120 x 2 = 240 Birds

Or........about 100 other scenarios, depending on how many pens, how many birds, etc.