For me it doesn't find much else leftover after using Revo Uninstaller, sure there may be little bits here and there but the majority of the leftovers it deals with on my system are registry settings which Revo doesn't always seem to find when using it in a saner/safer removal mode.
As for how big the tracking logs are totally depends upon how much something installs since it will track everything, I currently have logs ranging from 2.35 KB up to 3.91 MB.
After you've uninstalled something and used TU to remove leftovers you can configure it to automatically remove a log from a successful uninstall, or what I do which is manually remove them because as I've stated after using it I then manually remove any leftover registry data in CurrentControlSet (if it's security software), etc., and I need to look through the log to find out where in the registry I need to browse to. The reason I do this is because locations in the registry like CurrentControlSet are those which TU doesn't have the Admin rights to remove anything due to the permissions of those registry keys.
One caveat is don't use it when uninstalling drivers such as graphic drivers, etc., or else it could really botch up the system.
Never had it before, tested it a few hours ago, cant say anything bad about it, the opposite. Even if it was only a tracking cookie, it found 1 that even spybot, ad-aware(deinstalled by now, now as good as it was before in my oppinion) and even MBAM couldnt find. Also I noticed its fastness in running and updating, though scanning fastness was about the same as most scanners. As a additional scanner for spyware my rating: excellent.
The last time I used the portable version which was a 4.x version it created files in the 'User Profile\Application Data' folder, so in that case it really wasn't portable. I don't know if it has that same behaviour with the 5.x version.