<do you think you could make a split with one frame of brood, as long as weather is warm and no need for bees to keep larvae warm?>
If you don't have enough to keep the larvae warm, You can't increase the numbers in the split. Less than critical mass. Guessing an answer to your questions; depending on temperature, breed, foraging, etc. two medium frames with accompanying bees. One brood and one food. This answer is just to give you a place to start thinking. Really general.
Hawk