Changing the size of the icons gets a little more complicated. You will likely need to adjust the settings in the following CSS to see what works best for you, but the following will increase the size of all of the icons and the space where they go:
#viewer-options
{ text-align:right; min-width:700px;
}
.viewer-option-btn
{ cursor:pointer;display:inline-block;margin-right:50px;width:50px;padding:0;
}
You can either add the above CSS to same resource (resource-1.css) or press the "+" button to add a second resource. Make sure to rename it "resource-2.css".