I believe it has more to do with how you carry yourself and what you say than it does with your appearance. When I go into a bike store I already know what I am looking for. When I walk in I make eye contact with whoever is working there and say "how are you doing". They usually say "good, what can I help you with". Then I say "I'm looking for X bike", or "I need X part". I'm 6'1" 305lbs and have never had a problem.

Randy