How are you measuring the wheel? Are you measuring the width of the wheel on the inside or outside of the wheel lip? Measurements should be taken from the inside, as this is where the tire actually sits.
I would not put more than a 235 width tire on a stock 14" x 5.5", which is a common wheel. Even 235 is fairly wide for this wheel. My 225s bulge a good amount on my 14" X 5.5" wheels.
If it is a true 6" wheel, you should be all set for a 235, maybe even a 245, but I wouldn't go wider. It can create problems on sharp turns when the wheel compresses the tires. If the sidewall is already bulged, the wheel will have far less resistance within the sidewall to compress and could over-stress the sidewall. You do not want to create weak spots on tire sidewalls. Very bad.
(Please save the comments. I know of several people myself who run 255s and 275s on stock 5.5" and 6" wheels, but I wouldn't recommend it).
Just my $0.02