I just pulled mine out this morning to start shipping.I believe the largest is: inside dimensions-11 7/8" X 3 3/8" X 13 5/8" Hope that helps.
Bonnie

Thanks abbyrose. That's very helpful.

There are two shapes but both hold about the same amount of "cubic inches" volume.
The flatter box is as Bonnie said above.
The deeper box is 11 x 8 1/2 x 5 3/4. Just depends on what you are trying to fit, which one will work better

I know Flat Rate is often the best value for heavier things, and often just the most convenient, but according to my Priority Cost Sheet (I printed out), I can send up to 8 LBS to mailing zones 1,2, or 3 (also printed out) for $7.15, or 4 LBS to zone 4 for $7.45.

Having a scale (Ebay-Cheap!) and my printouts makes it really handy to decide what is the best value...especially if I'm paying the postage, as opposed to trading flat rate, for flat rate, or offering things up for postage.

Sue...on dial up so printouts are really handy.