How Big of a rear Sprocket

[wicked_silver]

I ride in Southern Ohio, we crawl around alot of hills down here. On my 400ex I already have a 14 tooth front sprocket and I would like even more gear. My question is "whats the biggest rear sprocket you can run on the rear and still keep the stock guard from rubbing? ( I would like to get a 40 or 41 tooth). Thanks in advance.

[remlapr]

You aren't going to get much - it doesn't look like there is much room. Are you running 20" or 22" in the back?