What box size and weight do I enter?

Weigh the packed box, measure it, enter exactly that. The shipping estimate the buyer sees and the label you buy when it sells are both priced from these numbers.

If you pack the same box often, save it as a box template and pick it next time instead of re-measuring.

Carriers re-weigh and re-measure packages in transit. Declaring low does not save money — the difference comes back as a shipping adjustment billed to you, with a fee.