Checkout
devel:css#user_styles
I put my changes in to conf/userstyle.css. Regarding the navbar height, the option you're looking for is:
.navbar {
height: 50px;
}
That's the standard/default. Change the 50px to whatever you want. But that's only for the navbar div itself, and there are many nested divs in there.
Checkout the documentation from bootstrap themselves on their navbar to see what classes you would want to adjust. There's also a not-doluwiki-specific howto
here since Bootstrap is used all over the web.