hmm, they are huge plants (unless you have a dwarf variety? but i havent heard of one)
my broccoli/calabrese plants were about 1metre high and had about 60cm spread. i did get a loverly crop of them for quite some weeks :)
if growing in pots, i would go for big pots (30cm diameter minimum) and plant them into a soil based compost/soil mixed with compost.
as for seedlings, i started mine in toilet rolls (they like it deep for their roots to develop) then transplanted into soil when they were about 10cm high.
hth
svea