I'm getting a margin size error message (saying too big) no matter how small I set the margin.

This message isn't exactly bogus; it just isn't very communicative. Here's what will usually work: On the Page Layout tab, in the Page Setup group, click Columns and select More Columns...

In the Columns dialog, chances are you'll see that the box for "Equal column width" isn't checked. When this happens, the column width doesn't adjust automatically to the margin width but stays fixed at a certain amount. Where you can run into trouble (aside from increasing side margins) is when you switch from Landscape to Portrait orientation.

The fix is to click the preset picture for One (make sure that "Apply to" is set to "Whole document"). This checks the box (which you can't do manually if you have only one column). If your document has multiple columns in some sections, you can change the number of columns back after you get the error corrected.

I'm glad you got your problems straightened out.