It will depend on the complexity of the document and available RAM. I can create a larger document but that's with a single layer. You may find that merging some layers if that's an option will allow you to enlarge the painting. If you have sprayed unfixed stickers, these can take up a lot of memory. If this is the case, going to your sticker layer in the layer stack, clicking the footprint icon and choosing to flatten to paint will reduce memory usage. If you have stencils in the document you're no logner using, removing these will free up more memory.
Resident Bug-Hunter and Support Guy