Start with this guy.
http://www.bobbrowncycles.com/story.htm
There was an article a few years back in the Star Tribune about this fellow at 400+ pounds who wanted a bike built for him, and Bob Brown was the only one willing to do it (complete bike). BTW the guy now weights 180.
You also might want to post your question on the Clyde Forum since you sound like you are concerned about your size.