The 48" should get you below the frost line, a couple more inches won't hurt.
The 24" hole gives you more wiggle room to square things up and gives more support to the posts by a bigger foot print. I'd go bigger than 12" as your posts should be 6x6 which gives you only 3" of play on each side.
I'm in Northern Ohio, on a building your size, we're required to have to have 20" wide holes with a minimum of 8" of poured concrete. No precast or dry sackrete. I'd pour the concrete and let it set a day or 2. I'd be concerned with dry mix not setting uniformly and thoroughly causing post to settle later on. You will get all kinds of advice on dumping it in dry and building. The problems show up later and are costly to fix. With the end of your posts below frost line and the weight of the building, heaving should not be a concern. Getting your post set, your skirt boards, girts, and top beams in place will minimze any twisting of the posts. Once you're sided and under cover, the posts will stay dry and should warp or twist. We backfill with #411 limestone, dampened and tamped. The limestone will set hard and shed water. My 2 cents worth.
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