it depends on a couple things. what head you choose and what kind of sound youre looking for. through my bass playing career ive played through 10s, 12s, and 15s. i am really partial to the GK 2x12 cab i have. you have a couple things to consider with cabs
speaker material:
ceramic, neo, aluminum?
the neo speakers in my cab are clean and articulate. however, they dont have that slight distortion a ceramic speaker would have. that is one thing to consider. ceramic speakers have that classic sound that everyone seems to want (somthing ive never understood. why do you want to sound like everyone else?). aluminum cone speakers have a brighter sound. ive only played a few cabs with those, so i dont know much about em.
speaker size:
10 inch speakers are going to give lots of punch and clear mids. 15 inch speakers will give you that low end rumble and can be a little bit smoother/muddy depending on how you eq your amp. 12 inch speakers are kind of in the middle. my cab has a very warm and smooth tone but still maintaining articulation.
you also may come across a ported cab. i dont really understand the science of it. but its an option
common cab configurations are 4x10, 8x10, 2x15. and lately there have been more 2x12 and 4x12 cabs coming out.
once again, find a store with a good selection and play through em all. your salesman may not like it, but make him hook up different amps to different cabs. dont just say "well this is what they have set up, so i'll just try those".