I am a little late coming into this subject. I hope someone can help me out.

As long as I use PBS or other authorized vendor printed postage, I can send packages over 13 oz w/o standing in the P.O. box line and physically handing the package to the postal worker.  OK. I've got that part.

But does anyone know how BIG (size, not weight) a package can be and still fit into the slot in the post office lobby? I suppose I could go down there with a tape measure or a variety of boxes, but I'd feel kind of silly. I am just wondering if anyone has an idea how wide and how tall a box needs to be to easily slip through the slot using that swinging handle door.

I'm thinking you may have to employ your measuring tape.  LOL. 

I'm thinking you'll have to measure the slot opening. I doubt if these things are the same size in every post office.

My post office lets us just sat the large packages on the counter and say to someone "electronic postage" without standing in line or going directly to a clerk.