I have laid out my 32-page children's book as 8x10 with .5in gutters for laser lines at printing. Then it struck me, perhaps I should have made the pages 9x11? Help? If so, then is it easy to adjust the size of the pages from my document or do I have to start over?
If you show more info in the document setup you’ll find the bleed setting.
Follow Bob's advice as to the Document Setup, assuming you do not have bleeds in the document your likely good to go.
To expand on that, always design to actual finish dimension. It's the un-seen meta-data that comes into play here. By designing at actual finished trim size, the trim dimension is included in the file data.
For example, you could later print from Acrobat to letter size paper, enable crop marks, and they would be properly positioned.