Hi, I can't give you an exact answer, but here's some pieces of information.

1. My wife crawled into bed (she was scrunched up tight, since the bed is pretty small) with one of our kids and it supported both of them. She's about 120 lbs and our son is about 40 lbs.

2. Our twins both weigh about 40 lbs. One of them excitedly jumped up and down on the bed the first night and ended up breaking a couple of the slats. Based on that experience I would not recommend this bed to anyone with a normal, active 3-4 year old. (Our kids used to bounce up and down in their cribs all the time and are bummed they can't jump on their new beds now.)

It's 53" LONG and 28" WIDE.

You could definitely use a bed rail as long as it's not longer than 3ft.

it says it uses a crib mattress in the description