I built my teardrop around a queen size mattress with about 1/2" inch extra on each side. I found alot of extra expense in the extra couple of inches on the outside materials wise.
The getting-the-thing-in-the-door is a factor too unless you are using foam. I am using a "real" mattress but I have an extra wide door. It WILL go in
I realized that my AC unit that sticks in at my feet was not going to be over mattress level after I lowered the galley counter not thinking about the interior repercussions, so I was forced to use a Full size bed rather than the queen size to compensate, but in the end I am happy with this as it will give me 3" on each side extra to change sheets, or build a little hidey hole with a pad on top.
If I would do again, I think 60" exterior width would be nice to accommodate a Full Size bed with 1 1/2" extra on each side. IKEA is a good suggestion for the foam ones.