you can ship boxes as First Class Mail packages

The maximum allowed size is a length-plus-girth of 108 inches!  As you probably know, the weight limit is 13 ounces.

But boxes that are TOO small can be a problem -- the USPS strongly prefers that the whole label should all be on one face of the package.  Really small boxes should be put into envelopes for mailing so they can fit the labels, and not get lost.