Size doesn't really matter as long as they are the same colour and same design. The printer can use the same screen for each shirt. As long as there is not a huge variation in sizing, like a women's small and a men's 2x, you should be fine.
So you would have 30 of each design/color combo, correct?
30 is still a relatively small number for screen printing, I would suggest going with a smaller quantity of designs with larger quantity of shirts per design, to be cost effective.
For example, 5 designs with 60 shirts each is going to be much cheaper than 10 designs with 30 shirts each, even though it is the same amount of shirts in the end.