I prefer using a .jpg image for my logo. My logo is 280px width by 110px height. One thing to consider is that in responsive view a larger logo like this will be too large on small displays. The XenForo code tries to scale logos for smaller screens but I have not found this to be effective. So what I do is use an add-on to remove the header in Responsive.
The logo XenForo uses is very nice and small and works great on mobile devices, but not many sites have such a short name.