:welcome2: to tPF!! I would say that there's quite a big difference between those three sizes. With the 25, it's cute, but not too much space to play with. You would have enough room for wallet, small agenda, cell phone, and/or small items of that nature. When you go up to 30 cm, it's more roomy. This is the size of my birkins, since 35 doesn't work for me as a handbag. I can get quite a load, such as wallet, agenda or two, cosmetic pouch or two, blackberry, and still have plenty of room left. The 35 has plenty of room. There should be enough room to carry files and even small to midsize laptop along with the essential stuff that we usually tote around. Keep in mind that as you go up in size, the bottom of bag is wider as well, so that allows even more room than the smaller size bag.
When you're asking about the space at the top, I almost want to assume that you could lose about a third or a fourth maybe IMO, if you strap and close it up. I could be wrong, just judging by a 30 cm, that is.