30/30 is a lean, mean, killing machine. I run that on like my filter freeps/forest nazzie deck. Cycle through completely by 5 and slam with 13 nazzies immediately thereafter. No, serious, I do. Consistently, which is the thing with smaller decks, they're consistent, you know how they'll act, you'll get cards you need, etc.
Between 30/30 and 33/33 is about my typical deck size. 34/34 and 35/35 is my size for decks that do a decent amount of cycling. 36/36 is about max for a cycler Dwarves freeps. 38/38-42/42 is the legendary Dwarf/Moria swarm.
AC (from the old boards) had a competetive deck, though, that was running 120/120. Course, he was using the
Gamling's Horn combo to pull like 20 of those cards out of the deck each site...but...ya.
So.
Combo decks, never more than 30/30 (typically with the freeps filtering- because shadow combos are insanely more powerful than freeps combos, which can still be outrun...unless you're like, talking fruit loops or something).
Regular decks- no more than 33/33.
Cycler decks- not typically more than 35/35.
Then specialty decks can do decent things as well.