queen_mab,
It sounds like your packages are pretty small, in general. You don't need to put in the dimensions of the package unless it is considered large (over 1 cubic foot) or irregular. Those boxes on the Auctiva page are there just in case you need them - you can leave them blank.
I noticed that when I only offered parcel post (or whatever it is now), the measurement boxes went away.
2purrcats