Well, the problem is, that changing the size of the logo is only a part of the job. You do also have to adjust the height of the navbar and the vertical alignment of the menu accordingly.
Check http://www.restaurant-roessli.ch
I'm not a developer, so I did that in very nasty way manually using the HTML-Editor (Logo size 100px):
Main Layout:
<body>
<div id="wrapwrap">
<header>
<div class="navbar navbar-default navbar-static-top" style="height:120px">
<div class="container">
<div class="navbar-header" style="margin-top:-17px">
<button type="button" class="navbar-toggle" data-toggle="collapse" data-target=".navbar-top-collapse">
<span class="sr-only">Toggle navigation</span>
...
Show_logo:
<data inherit_id="website.layout" customize_show="True" name="Show Logo">
<xpath expr="//header//a[@class='navbar-brand']" position="replace">
<a href="/" class="navbar-brand logo">
<img src="/logo.png" t-att-alt="'Logo of %s' % res_company.name" t-att-title="res_company.name" style="max-height:100px"/>
</a>
</xpath>
</data>
Anybody willing to automate that and publishing a small module, which allows to enter the required logo size and then adjusts everything automatically?