How to make Font size equal to 15 in Terminal on Mac OS X Yosemite?
Here is what I did in OS X Yosemite 10.10.4.

Terminal (menu) > Preferences... > Settings > Select the target Profile > Text > Change... (button)

This brought up the Fonts window shown in the image below.



I then resized the Fonts dialog box vertically until the Size text window appeared and entered 15, as shown in the image below.