Liquify

Liquify is a  tool you use to make something bigger or smaller. To get to liquify in photoshop you go to Filter --> Liquify. Or if you are on PC just press Shift + Ctrl + X.

A new window will open and these settings will appear on the right side of the screen

The most important tools you want to remember is the one that is marked blue and the one above. The blue one enlarges what you press while the other one does the opposite.

To your right you can see this box where you can choose the size of the brush you want to use and how much pressure you want to add to it.