I dont know John, but you're way ahead of me. Mine are only just starting to grow. Its been so cold up here that they have just sat for weeks in their compost doing nothing! Hope they do well for you though, and if they are a bit 'leggy' i'm sure they will sort themselves out once they are planted outside where the light is more consistent.

Hi, there are about 900 species of Begonia with varying growth habits, and lots of cultivars, I'll presume that you mean the tuberous rooted species, I think 6" is about right, as long as they don't look 'forced', but they wont be ready for planting out until all risk of frost has passed in your area, Derek.