The main difference between M50s and M60s is that the 60s have a dedicated midrange so vocals are a bit more "forward" and clear in some rooms. On the other hand the 50s are supposed to have a more relaxed sound and are probably more tolerant of bad recordings. It's hard to say one is "better" than the other (unless you own one or the other, of course )

Size wise there isn't a huge difference, ie don't worry about room size between 50s and 60s.

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8