Mail Merge and Changing Font Size on Envelope

I am trying to print Christmas envelopes. I have set up a database in Microsoft Excel and I'm using MS Word Mail Merge to print the envelopes. Everytime I try to increase the font size greater than 12, the address moves and will not fit in the space. I have decreased the margins to almost nothing and also on the Excel worksheet, I have changed the column widths with the largest one being 25. It looks like it should fit, like I've got plenty of room. What am I doing wrong? The font really needs to be a little bigger.