As said before...6 feet is plenty wide BUT you'll find that a 7 foot wide trailer makes life so much easier~!~
Depending on how you strap it down, you'll notice you have more leverage to hold it safer and also less stress on your plastics (if you go across the plastics like some do).
Mine is 8 foot wide inside, and I use canyon dancers, then go across the plastics as a back up and use seatbelt cushions from Walmart over the straps to help protect the plastics.
No matter how you strap it in, if you get a good sturdy wheel chock it makes all the difference in the world~!~
Anyway, 6 ft is fine but 7 ft is much better...plus leaves you a little breathing room.
My last trailer (sold it) was an enclosed 7x10 and worked out wonderful~!~