those look big enough to me to allow them to bloom.

I've never found it necessary or wished to nip off buds to promote bigger bloom, I've always had to nip back leaves that hide the color.
This year is sort of strange, I've never had the good fortune to have my starts so advanced in size and yet I've always used the ides of March as my starting date.
This year all six of my potted begonia are up much more than I've been used to...fully 3 of them are up about 6".
Its very encouraging to think they will produce more glorious color this year than past years.

Since I took a bunch of seeds from various plants and just sowed them all together in a box, I have no idea what colors my flowers will be. Thanks, gladzoe; I didn't pinch and I'm glad I didn't, as the buds are growing slowly, and I'm anxious to see what colors they will be!