24' is a pretty good span. Unless you are using TJI's, you are looking at a bearing wall in the middle downstairs. Per the TJI span table, you are looking at 14" or 16" TJI's to clear span the loft, and possibly web stiffeners.
If you use 12' in the front, and want 4' sidewalls in the back your ceiling height is 8' less the height of your second story joists and subfloor. That makes for a low ceiling downstairs in the back, or loft sidewalls less than 4'.
If you use 2x10 across a 12' span (assuming you place your bearing wall in the exact middle of the downstairs back-end, you would have to build a small pony wall on the deck of your loft to line up with the top plate of the 12' walls. To tie the two sections together, I'd think you would need a top plate that spans the entire length of the 24', with one piece spanning the two sections to tie them together. Even if you use TJI's, you are still building a small pony wall on the deck.
The roof plan now becomes an engineering issue, as you need to accommodate the outward pressure across the pony wall section. You are looking at trusses or an engineered ridge beam, as opposed to a simple ridge board.
The pony wall is also a standard 2x framed wall. You will end up with a 24' top plate spanning both directions to set your roof rafters. With a 24' width, I think you are looking at attic trusses or scissor trusses. At the width, you would still end up with decent height in the center, even if you only have a ~3' pony wall to accommodate 16" joists (assuming you want to clear span). Trusses would be much easier to install and handle.