titlebar.xmlPut this in the generic folder. There are 3 lines you'll need to change.
Line 31: <container height="50">
Line 48: <widget class="picture" id="lgtF" scale_picture="true" keep_aspect_ratio="true" image_alignment="centre" width="36">
Line 49: <layout class="stick_to_sides_attachment" alignment="vertical" inset="6" />
Reducing the inset value on 49 should make it a bit bigger, though if its not enough, you'll have to edit the other two. If you add 4 pixels to line 48, add 4 to 31, and so on.