Any updates on this in July 2020?
It looks like a simple basic functionality that one should be able to draw shapes of precise shape and position it on the canvas -
through expression where needed, rather than challenging manual labor.
Probably through a Properties panel associated with an object (of course it remains as an object does not become pixels).
A simple task I started to create a YouTube Channel art requiring four rectangles of sizes
2560 x 1440
2560 x 423
1855 x 423
1546 x 423
all centred on the canvas