techincally anything over 35 minutes (might be 30, i can't remember) is a full length.
that being said, i have to agree with pg666 with an exception. i'm not a huge fan of EPs. but to me an EP IS BETWEEN 3 - 6 songs and usually leaves me unsatisfied with a good picture of the band.
i don't think you can justifiably charge 10 bucks for an "EP" and that makes some of the economics unappealing to me for my own band.
what i agree about is that a lot of full lengths are way too long. just because you have 15 songs doesn't mean i want to sit through them.
either do a few singles, save some songs for the next record, or just scrap em all together.
8 - 12 songs is about my limit depending on how mars volta you want to get with song length.
personally, i'd suggest finishing all 12, have them mix to your satisfaction, and then sit down with a good sequence and make a decision.
if you don't like the 12 together, you can alway pull a few (still have an LP to sell for 10 bucks) and have a few finished, mixed tracks to use for random things that come up.
it's not unusual for me to mix a full length of 15 songs when only 12 were ever expected to be on the album. since i typically mix an entire album for a flat fee, most bands just send everything they cut and just get it done.