You want to have a mummy/sleeping bag that will fit the climate you plan to be in. If you plan on being in sub-zero temperatures (snow camping), you want something like a 0 degree bag (not to mention a sleeping pad to keep your body off the snow). If it's just for summer camping in a place where it only drops down to 55 deg, the 45+ will be fine. Nevertheless, plan for the coldest and go from there.
I have a 15+ for general backpacking trips and it's super warm. Sometimes, I'll unzip the thing to air out because it gets so hot. If you get a 45+ and plan on going somewhere a little colder (I'm talking 10-20 deg difference), they do make liners that are compatible with certain bags that help with warmth (not to mention fleece liners to up the coziness factor).
As for compression size, click on the "specs" tab when you're on the REI Product Page and scroll down to "stuff sack size". That will be the size of the bag when it's compressed. If you need it a tad smaller, you can get a compression sack that could probably squeeze it down a bit more.