Size differences between the the P series SIG's are minor, unless you go with a P230/232 or the P238. Even the difference between the single and double stacks is minimal. Except for the .380's, they pretty much all carry the same.
It seems a lot of people consider them to be "large" pistols, but large compared to what? The P220/226 are Colt Commander sized pistols, which I dont really consider large, but then again, thats what I carried, along with Government Models most of my life before I switched over to SIG, so I dont see any difference. Maybe if your used to carrying a J frame, or Kahr, etc, they seem big, so its probably more of a perception thing than anything else. I've never had any problems concealing any of them under an untucked oxford type shirt in the heat of the summer.
Caliber wise, I switched from the 1911's to a P220, as the .45 was what I was accustomed to, and felt it was the best choice at the time. Whats not to like with a P220? Great trigger, accurate, and easy to shoot well with.
Not to long after, I got to shoot a 357SIG, and was quickly converted. Havent carried a .45 since. Switched over to P226's and P229's, and after mostly carrying a P229 for a number of years, I've switched back to a P226, simply because it has an extra 1/2" of barrel length that maximizes the 357SIG's ballistics. You get almost another 100fps out of that extra 1/2". Over the .45, the 357SIG gives you the extra rounds of a somewhat more potent caliber in a high cap gun, thats basically the same size as the .45's, so whats not to like.
I have spare .40 barrels as back ups for each gun, but other than checking them for function, I rarely use them, and except for one, the others dont get used at all. Its nice to have the capability to switch if need be, but I'm thinking I'll never see the need at this point.
Your best bet if possible, is to try to shoot what you think you might want before you buy. The difference between the P226 and P229 is really nil, but at the same time, they feel different in your hand and how they present. Same goes for the P220.