This article is helpful in stating options - the different considerations you should make before committing to a purchase.
Im a very visual person. So, I would arrange my furniture as I like it best. Then I’d lay bedding on the floor to mimic where the rug would lay. Crazy, but that’s me Easier than hauling a heavy rug in and then back to the store.
Standard large rug sizes are 9x12, 10x14, and 12x15. If you’re looking for old oriental rugs, there are often more variations.
use painters tape on your floor to mark out the area you want covered, with your furniture arranged how you want it, then measure the tape, doing a "dress rehearsal" like this lets you be confident you're getting just the right size
Depends if you want the flooring (like wood) below to show thru. If your floors are in bad shape, you can have it cut to exact size, and have the edges finished off. I have one room , where it is cut right tot he woodwork. Also consider where you wan the furniture. If there is a small space, it can make the furniture arrangement awkward.