The Line tool should work for you, I suppose. Just hold down shift while you drag to get a perfectly 0, 45, or 90 degree line.
As far as judging distance goes, in the bottom left corner of the Paint.NET screen it tells you how many pixels you're stretching out your line - so if you already know the reference distance to another subject in the pic (like a standing 6' man) you can compare it.
I hope what I just said at least attempted to answer your question