Hi,
should be very easy...
1. you can set the icon size (for example font awesome) with: fa-2x or fa-3x or fa-4x.... (
http://fortawesome.github.io/Font-Awesome/examples/ at point 'Large icons')
2. t3 is based on bootstrap, so just use the helper css class 'clearfix' with responsive visibility 'visible-lg visible-md visible-sm' (
http://getbootstrap.com/css/#helper-classes-clearfix) to bring the text on the next line
Ok now to the praxis... Just go to the configuration of mega menu (picture 1) and add an icon with 'fa fa-home fa-4x clearfix visible-lg visible-md visible-sm' (picture 2) to your menu and enjoy the result (picture 3).
Of course you can modify (styles) this how you want.
Regadrs,
Eugen