For the best results, work at a suitable resolution for the intended end use. I have a printer that can print up to A3+ (329mm x 483mm - 13" x 19"). I've created document presets in Painter for that size at 300dpi. In pixel terms that means that when working on an A3+ document it's actually 3886 pixels x 5705 pixels.
Many people will start with their document size much smaller, and resize it upwards as they go along. Waiting until the image is finished and then resizing it is probably the least useful solution.
I'm not sure what size your document is to begin with, but for print quality your 24" x 36" document would be 7200 pixels x 10800 pixels (that's at 300dpi - at 150dpi, which might give you acceptable print results - depending on what you're printing on - you'd only need 3600 pixels x 5400 pixels).
The further away from these sizes your document is, the worse it's going to be resizing.