<font color=blue>...dealer keeps telling me to get a 5' brushhog no bigger???...</font color=blue>
I agree...
I think you'd be much happier with the 5' with only 30 PTO hp... it's a better implement/hp matchup especially if you have substantial brush or tall grass... it won't bog down as often and you can zip through the work at a good pace...
Here's a case where matching the size of the implement to your wheel width may hinder more than help...