3 1/3 X 4 chassis: How tall can I make my trailer?

Hi, I would first like to thank everyone for the useful info I have read during the past week since I've discovered this site. You've really got me brainstorming. For my first experiment, I would like to make a square tailgating trailer which would basically store and organize all of my supplies: a grill, cooler, etc. (no bed) on a 3 1/3' wide by 4' long chassis (anything larger and I would be charged for an extra parking spot). The trailer is this one:
http://www.harborfreight.com/cpi/ctaf/D ... mber=42708
Anyway, given it's small dimensions, I was wondering how high I could build the trailer without the chance of it being unstable and tipping over on the highway? What do you all think? Thanks for your help.
Steve
http://www.harborfreight.com/cpi/ctaf/D ... mber=42708
Anyway, given it's small dimensions, I was wondering how high I could build the trailer without the chance of it being unstable and tipping over on the highway? What do you all think? Thanks for your help.
Steve