I imagine very few of the windows you'd pass in that mission are even interiors. Most likely they're a diorama of sorts, and can't even be accessed.
The interiors will be as big as they need to be, and likely no bigger. Rockstar isn't big on wasted space. With the exception of the various shops, there weren't any interiors in IV - that I can think of - that weren't used in a mission, so if V does the same, then they'll only be big enough for what they're being used for, plus some added things to make the place appear bigger, like the 'dioramas' above. There will be other spaces (not necessarily interiors), I'm sure, like the outdoor mini-mall in San Andreas.
I would like to see the end of interiors that can only be accessed in one mission, like the bank in IV, or interiors that can only be seen in a cutscene, like the prison visiting area. I should be able to go there any time I want.