Krita has an efficient technique for storing parts of the layer which have literally nothing on them so you can do that and have a small .kra file.
Sometimes, I make a 2048 x 2048 image as the intended final size, flood fill it with green (say) then set the opacity to 10% then Resize to 4000 x 4000 dragging the red (current size) rectangle to the bottom left.
Then I have a 4000 x 4000 canvas with a pale green overlay square to indicate the ‘final product area’. That can be turned off of course.
Then I’m free to doodle and construct or whatever in the ‘white’ areas and then copy stuff into the ‘green’ area to compose a final product.
When I’m satisfied, I Save, then select the green overlay layer, then Trim to Current Layer, then Export, then Close without Saving (very important).
If I was sensible I’d do the final Trim on a copy of the saved .kra file.