Are the proportions off or is the aspect ratio off?
As an example, let's say you are mapping to an object that is 2 foot high by 3 foot wide. When you go to full screen is the problem
(a) that what you project is 4 foot high by 6 foot wide - so it is in the right ratio, just double the size?
(b) that what you project is say 4 foot high by 8 foot wide - so the ratio is altogether wrong (it isn't a multiple of the original)?
I've had both happen when I first started and I'm still a beginner so take my solution with a grain of salt. But here is what I did to fix each.
In the first instance, it was just a matter of zooming in and out on my projector to get it to the right size (or, physically moving my projector forward or backward if feasible and needed).
In the second instance, which happened frequently when I started, I was having difficulties with the output size on the projector (find this under the projector tab in MadMapper) and the aspect ratio of the projector itself not working together correctly.