I ride a small frame (49cm) , and with the largest seatbag I can fit (which barely clears the top of the rear wheel by a centimeter), I can fit my windbreaker. If I roll them up tightly enough, I can probably also fit some leg or knee warmers in there. Of course, this means everything else (money/atmcard, gels, phone), have to go into my jersey pockets or a bento box. If there are plenty of water stops along the way, and you can get away with only one water bottle, you can stuff a 2nd water-bottle with other supplies as necessary. I once stuffed my windbreaker in a waterbottle.