For sake of completion/helpfulness this program is called Paint Tool Sai, there are English versions on the web.
Add 100px to the width and 150px to the height. You can do this from here. Don't forget to do so from the center.
In image editing software, commonly while adjusting the width/height you can choose an "anchor point" from which to do so. Ryou-san is referring to this (meaning make sure your anchor point is the center).
