Below the command box click Customize (Keyboard shortcuts).
On the left side we can see different groups of commands.
I chose All commands, to get a list of all supported commands.
On the right side now we can see a list of commands for selected category.
I chose GrowFont (this key combination increases font size for one pixel).
Below you find a box Press new shortcut on the keyboard.
Click on the empty field and press the key combination for the desired command.
I chose Ctrl + <
We can see that our shortcut was added to the list.
From now on, if you have highlighted text and you press the key combination Ctrl + <, the font size will be increased by one.