That's what I've thought about doing. The user can always change it to dialog units if they want, but if they use the builder, then it'll have to be in pixels.
That's what the builder will look like.
Suggestions welcome.
Edit: I really doubt the feasibility of this, but I'm going to ask anyway. Is there a way of working out the co-ordinates of a rectangle drawn with the mouse onto a Plus! window? Would this need a certain control? If this isn't possible, would I be able to record two clicks (one for the top-left, one for the bottom-right)?