Here are the configuration settings for the Terminal app in Yosemite.
It shows that I can set font size to the distinct values given in Terminal. How can I set it to some other in-between value like in my case, I want to set it to "15"?
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.