The data is easily available, and sizes are in inches and millimeters (not pixels)...it's the relationship to the pixels and such that I'm unable to figure out...

When I draw it I just scale it to the size of the window - whatever that may be - I don't calculate the size beyond that. In this case, for sizeHint I want to be able to go one step further and actually be able to make a size calculation. Yes, the scaling matrix will help keep aspect ratio, but that's about it.

How do I actually go about making a guess to the right size?

For example, the layout of the first dialog I am using this widget in uses a QVBoxLayout manager, with a slider on top, and then my widget below it. I'd like to put the widget into a QHBoxLayout with horizontal spacers on either side.

I could probably access the parent and get one of its rects, but that would give a larger area than I would technically have. Or I could loop through all the children of the parent widget that aren't me and get their minimum sizes, etc. and figure out a size, but then - isn't that the job of the layout manager?