Go with an 8x16 if you can. Check my link for some of my pics. I've got a 36 and two 21s side by side with my 48 up front. If I put the 48 in the back, I would barely have room to move. My trailer is a brand new 7x16 and I already wish it was 8' wide.
Two things to add on if you can afford to order one: 7' interior height (so you can hang stuff from the roof if need be), v-nose (slightly better fuel mileage, but more importantly, more storage without impeding on your floorspace)
I added the side ramp for more efficient loading/unloading and it will handle any mower up to 60" wide.