How to change the font size in Spacemacs

Open your .spacemacs file that resides in your home directory:

Look for this section:

dotspacemacs-default-font '("Source Code Pro"
                            :size 13
                            :weight normal
                            :width normal
                            :powerline-scale 1.1)

Change the size and restart Spacemacs.

How do I change font inside Spacemacs using shortcut keys?

Here's a great article that shows how bind control + and control - to change font size.