The settings that need to be changed are located in Default.sublime-theme. I'm using Fedora so the file is located at, [HOME]/.config/sublime-text-2/Packages/Theme - Default/Default.sublime-theme.
Add the font size to "sidebar_heading" and "sidebar_label."
{
"class": "sidebar_heading",
"color": [130, 130, 130],
"font.bold": true,
"shadow_color": [250, 250, 250],
"shadow_offset": [0, 1],
"font.size": 16
},