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.
Optionally, you can increase and decrease font size for bash with,
Command and the Plus (+) key Command and the Minus (-) key