I think you are misunderstanding me. I fully understand that Xara is a vector, not a bitmap, editor, but you can still set the page dimensions in pixels (a "pixel" in this case, presumably being an arbitrary unit based on the dpi of the page). Animation docs default to having pixel-based page dimensions.

If i have a page that is set to 1000px x 1000px (you can do this in the page size tab, of page options - just change the units to "px"), and i hit export png, it defaults to export "drawing", but if that drawing is say, only 75% of the page, it will attempt to export as 750px x750px, which is what i'd expect.

If i then click on "page", i would then fully expect it to export the full 1000x1000px image, but the boxes where you enter the export res don't update.

I can even confirm that this is simply a bug - i just realised that if i switch to "page" , change to another export tab, then change back, the export resolution boxes do, in fact, update to the expected 1000px x 1000px. (this is actually easier than typing the export res by hand, but still an unnecessary step)