No amount of fiddling would have it do the whole set. That meant that if I wanted Slovenia, I'd not have been able to get the UK and Holland in. I'd set that aside as a problem to deal with soon.
The default options for areas of mapping to download to the unit, should all of Europe not be able to fit on, are somewhat limiting in my experience. ISTR that the 765T (I have one of those as well as my SP2720) has 4GB, or thereabouts, of memory. Fitting Europe into that shouldn't be an issue for many years to come. I assume that, being from the same family, that the 660 has the same

Maybe you had loads of MP3s on it last time around?
If you do have issues with not having enough room you aren't stuck with the default options for mapping areas. Downloading the mapping to your PC and properly unlocking it there will allow you to generate a custom primary mapping file, selecting the mapping tiles you want and sending this to a suitably sized memory card or USB stick from within Mapsource. This will be called 'gmapsupp.img', all you do then is rename it to 'gmapprom.img' and transfer it to your unit. Best to save the existing 'gmapprom.img' to your hard drive first though. All the map generation and transfers can take several hours though.
For Garmins that don't have the ability of being read or written to as an external drive, such as my SP2720, you can use some command line code to send the mapping to the unit, though there's no way of backing up your existing primary mapping.
Here's how (long thread).
Please note that this isn't a means of getting pirated mapping onto a unit, the mapping needs to be unlocked on your PC with the unlock code specific to your Garmin GPS. With the more recent versions of Mapsource this must be done on-line with the GPS connected via USB at the same time.
still awaiting the download, yes, 24 hours later due to Garmin's servers and Pipex/Tiscali's rubbish speeds it's still only a quarter of the way through the 3632Mb download.
By today's standard that's a pretty appalling connection you have there

On the few map downloads I've done previously the bottle-neck has always been my connection, not Garmin's.