I'll spill some of my knowledge between the 3 different carrier versions.
It all started with the original International Galaxy S 2. This is the one with the rectangular iPhone-esque button on the bottom middle. (I wish it had kept this button, but I digress.)
From this phone as a reference model come the 3 carrier variants.
AT&T: This version is the closest to the original International Reference. The only main differences are the exclusion of the physical button on the front face, and the addition of NFC hardware via the battery. (The NFC is turned off via software though.) AT&T also customized their software on the phone just as all subsidized phone carriers do.
Sprint: This version has a larger screen, 4.5in vs the original's 4.3in. Lastly, because of the bigger screen it has a bigger battery. The additional battery size helps power the larger screen but in a direct comparison I believe battery life ended up slightly worse vs the original. This battery also does not have the NFC hardware built into it. Oh, the sprint also has a LED for messages on the phone.
T-Mobile: This version is the most different and has a completely different processor inside, I believe it was a Qualcomm dual core SnapDragon instead of Samsung's in-house made dual core Exynos. I'm not sure why this change was made though, as the resulting performance seemed to be about the same from what I remember. It has the big 4.5in screen from the Sprint model, but the original battery from the AT&T version. I believe it has the NFC hardware. It also has a very rounded off shape which I find unappealing. Sorry, you get some of my bias with this write up.
So to return to your concern over the size, since it seems that you are looking at the T-Mobile variant, please take my review of the phone's size with a grain of salt. The Sprint/TMobile phone is BIGGER than the AT&T phone.
And by a decent amount. I looked at the Sprint version in the store since it was released a week prior and it was too large for me.
IMHO, the AT&T is probably the upper limit on phone size that I find acceptable.