I recently bought a Trek 7.4 FX fitness bike and had a quick question on the u-lock size I should get?

I do not live in a high crime area, and am looking to lock my bike when I commute or run errands, but was wondering if I should buy a mini u-lock or standard size u-lock? I did hear that a smaller u-lock is better, but does restrict where you can lock up the bike.

Does anyone have any suggestions?