Size doesn't have much to do with it unless you are unhealthily under or over weight.
For reasons as to why you are not getting pregnant you must see a doctor. If you have been trying for 12 months or more you should see a doctor anyway to rule out causes of infertility (both of you).
If you had sex with more than one person at any point around ovulation there would be a 50/50 chance either way of the parentage unless one man has some condition that makes him less fertile.
Again only a doctor conducting tests on both of them can give you an answer on that.
However, if you have changed partners, still sleep with your ex and don't have a stable relationship/income/home/etc I wouldn't recommend you even consider bringing a child into this world.
How old are you?