This Scribe shows you how to set margin widths in Google Sheets. Note that a Google sheet won't have a visible margin until you decide to print.
Navigate to your Spreadsheet
Click "File"
Click "Print"
On the dropdown on the right panel, you can select between default [[Normal]], [[Narrow]], and [[Wide]] widths
If you'd like to go with a custom width, click "Custom numbers"
Number fields will pop up for each margin. Customize to your heart's content! When you're done, click "Next"