You can change the "Messages" tab using the zoom control within that tab, however the same is not true for the "Results" tab.
One way to change the font size is via:
Tools > Options > Environment > Fonts and Colors
Here you can configure the fonts for every aspect of SQL Server. In your case select "Grid Results" from the drop down menu, then change the font size. You probably also need to close and reopen SSMS for the changes to pickup.