Grow up the menu button

I don't think there is a way to do that without hacking into the dashboard core code, which is probably not a good idea, as it would be overwritten when you next update.

You could however hide the titlebar by Sidebar > Dashboard > Site > Options so that the default nav icon is hidden.
Then create you own menu system where you can use whatever size, shape or colour as a navigation button.
See Contextmenu location - #45 by Paul-Reed