Write the size as length × width × depth, in inches, in that order. Length is the front panel, left to right. Width is the side panel, front to back. Depth is top to bottom. Do not swap width and depth. A 4 × 2 × 6 carton is not the same box as a 4 × 6 × 2.
Measure the inside of the finished carton unless you tell us the numbers are outside. The flat blank is larger than the finished box because of the glue flap and the tucks, so a dieline measurement is not the order size.
A hang tab, a window, or an insert is extra. A sketch or a sample is enough to start the quote.