There sure is - when using icons in text (e.g. like in the skillbar tutorial), you can control the size via Icon Size text codes, e.g.: #iconsize#w100# will scale icons by their width to a size of 100.
However, the whole setup has gotten a few updates over time, you can now have more control over the icon in a skill bar (shortcut slot layout). In your Content Label defining the icon text, you can use the Icon type and use the icon directly, including defining the used size, etc.