I've been told so long as you have around 30-35mm you can trim them down to that. I'd guess you can probably trim them a little more than that depending on original size and if you re-use the stock ones or not. My coilovers came with replacements front and rear.
09 BSM E92 335xi||Black Dakota||Dark Glacier Aluminum Trim
313 Perf. Wheels||LUX H8's