IMO 17 is the best and 18 is the largest. 19s can be made to fit with slight modification to the fender lip. 20s are the same, but just look stupid on the e36.
Most 17s offer the same contact patch as 18s and weigh less. The disadvantage is more sidewall flex. Given there is very little sidewall with 17s anyway, the weight pro outweighs that con.
<span style='color:gray'><span style='font-family:Courier'>1995 e36 3 Series RIP
1999 e46 3 Series BURN IN HELL
2000 MR-S Roadster= Current</span></span>