I couldn't find the pixel dimension for the service desk logo (not the portal logo). You have the option to chose a logo under Settings - Portal Settings
Screen Shot 2015-02-24 at 10.50.57.png
In the popup window only hints for the file type but not the size are give:
Screen Shot 2015-02-24 at 10.51.06.png
It looks like the logo shows up as 64x64 when visiting the Portal as a Customer (ie: http://localhost:8080/servicedesk/customer/portal/2). When at the Global Portal landing page, (ie: http://localhost:8080/servicedesk/customer/portals) the logo is scaled down to 32x32.
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