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?