Usually would use a 3/8" round over bit. I have seen people use a 1/4" on the outside of the box sometime because that's what they like. But if you are rounding over both sides of the wood then definitely a 3/8". If it's on the outside of a box you can use what ever you like or on anything else.
I use 1/2" because that's all I have. A 3/8" bit gives a perfect half circle on MDF if you round both sides. If I try to do that with the 1/2" bit, it is more of an oval shape. They will both work and it's really up to what you want done. The price was cheaper on the 1/2" bit so I bought that instead of the 3/8".