Get a 102" wide and I recommend a 22' long box, a little more room for stuff ahead of the car wheels, tires, jack, chairs, etc. A V nose will save fuel and gives you a bit more room up front.
I haven't used one with a left side door but my Featherlite is wide enough that I can get out the car window. The main problem is the door hits the inner fender box so it only opens 4 to 5". We park the car favoring the right side, maybe gain 2" room.
Get a beaver tail, helps car clearance. A step down on the right side door is a big help. My older Featherlite does not have the step down & I wish it did.