Broccoli & cauliflower seedlings

Broccoli & cauliflower just germinating inside. How large should the seedlings be before I plant them? Would a hot frame be enough protection for the seedlings (some nights as low as 30, days typically 60 & sunny)?

Thanks,

Frank

Started mine today in a cold frame, will transplant late March. Here is a photo from the past. Broccoli plants on the right are ready for transplanting,

I like to get them set out by 4 weeks or sooner a bit from sowing, I think that it is good to get them in the ground fairly young.