Most people think 16" is the best size rim for an E30, it offers the best balance between looks, handling, tire meat, and weight. Also, some people think 17" and bigger looks out of place on an E30 because the car is older and more classically styled. A 17" should be OK if you watch for potholes, but don't go any bigger than 17". A 15" or 16" would probably be ideal, and 17" if you really want bling (and generally drive on smooth roads).
14" BMW (?) Bottlecaps
14" BBS Basketweaves
15" Ronal LS
16" BBS RX
17" Alpina Kopis
18" (I don't know the kind of rim)
Note: none of these are my cars
Edit: don't put low-profile tires/big rims on your car until you lower it. Aftermarket rims on a stock suspension looks foolish (unless you are gonna rally it of course
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