What size for a mother/clone cabinet?

I plan on using a 400w MH for my clone/mother cabinet. What are the optimal dimensions for this use and lighting? Of course i'll have a intake and exhaust vent for heat and etc. Would a 250w MH be better for this use and to control heat? The light is about 6 inches tall, and the container will be about 10 inches tall for the mother.

Thanks.

yeah . you can go for lower wattage for your mother plants but it's really up to you. i know you can safely put your mothers under CFLs with which aren't as good as your 250w MH. actually the 250w would be perfect.