My regular wallet is jersey sized and comes with me on all rides, in addition to my Road ID. My seat bag (which is smaller than the one I had before, but broke) is a bit smaller than I like, but I am constrained because of the small size of my bike, the teeny teeny bit of seat tube I have, and the modular shape of the tubes in general. I have 2 tubes in a baggie with powder, a co2 inflator and 2 cartridges, 2 levers, some band aids, antibiotic ointment, and a couple of tissues in a baggie. I can usually squeeze in a Luna Bar and a couple of Nuun tablets or Shot Blocks, too. Friday, I took the Luna Bar out and put it in my jersey and was able to squish my vest in there.
I try to buy jerseys with deep pockets. The best one i have so far is one of the TE design contest jerseys I got last summer. You could put the kitchen sink in there! But, when we are doing a longer ride, like today we took lunch, DH has 2 sizes of Carradice bags which we use. I wish I could put one on my bike...
2015 Trek Silque SSL
Specialized Oura
2011 Guru Praemio
Specialized Oura
2017 Specialized Ariel Sport