For two... five wide. For solo camping, 4 wide.
I've built and camped in several 5' wide Benroys. My first, a woodie, has a queen mattress with <2" cut off the width. For my son's foamie we lengthened the axle so we would have 60" inside and 64" outside (2" foam walls). So five wide is very roomie and comfortable for two.
I am currently building a 4 wide foamie. I solo camp and my goal is to make this thing as light as possible and stash the minimum amount of gear in it. I'm considering cutting the mattress width from 39" to 36" to increase the storage in the sideboard that runs head to toe on the road side. It will be a modified Benroy profile and primarily pulled with a small SUV EV. I hope I've made it light enough, narrow enough, short enough and with an efficient shape to minimize the reduction in range of the car. We'll see how well all that turns out.