First question: have you ever been camping or hiking much, or any, in the past? This is the best place to start with, for our understanding who we are giving advice to.

Which sleeping bags fit in your pack is of minimal importance until you know what temperatures you will encounter. You're going to have a wide variety of choices no matter what the answer to that is and their costs will also cover a wide range. Eventually, how bulky it is will be a factor, but not to begin.