How to increase font size in Vim editor?

This may not be an ideal solution but it works for me.

Just zoom in on your terminal with Ctrl+Shift++.

Zoom out with Ctrl+-

According with http://vim.wikia.com/wiki/Change_font:

Console Vim uses whatever font your console/terminal is using. [...]

When running inside a terminal, Vim can, at most, change the colours (within the limits of the colours supported by the terminal: sometimes bold and unbold black and white, often 8 colours plus bold/unbold foreground only [or seen another way, 8 background and 16 foreground]; on X11 some terminals support up to 256 background and foreground colours; "changing colours" usually also includes the use of reverse-video), and, if the terminal supports them (not all terminals do, and even those which do may support it only with certain fonts), use bold, underline and/or italic.

That being said, if you want to change the font size in Vim editor, you have to change the font size of your terminal. To do this in gnome-terminal go to Edit → Profile Preferences:

Moreover, you can save these preferences in a new terminal profile and when you are start using Vim, use that profile.

Ctrl + mouse scroll up
Ctrl + mouse scroll down.

This works for most terminals.