to be honest, it's not so much about size, but about body quality.
you could be a size 16 with a toned, firm and sculpted body and look great in a bikini, or a size 6 who's pasty and flabby and wobbly and look less than great.
you sound like you'd look good in one - size 12 is a good medium size, and you say you're not flabby and you have boobs, so i think you'd look great! kelly brook is a size 12 and looks, well, better than pretty much anyone else in a bikini so wear one and have fun! you'd probably feel more comfortable in one than you do in a tankini anyway, as you say you don't look that good in a tankini so probs don't feel that comfy.