I got a 60L backpack (frontloader) and I love it, but I sometimes feel it's a bit on the big side. I'm a light packer either way, so during the last few trips I found that there was a lot of empty space in there. I guess it makes it easy to find stuff, so it's not so bad.
The reason I got a 60L one instead of a smaller one is that I do a lot of hiking, camping and canoeing - then you DO need the bigger pack to fit your tent, sleeping bag, food and clothes. Personallt I think that for travel around Europe 50-55L should be enough as long as you are not gonna try to pack your entire wardrobe in there with you. It's amazing how little you really need to have with you when traveling as long as you think things through and plan ahead.
for more info for fitting a backpack. And remember: a pack is a personal choice - YOU will be wearing it so YOU should try it and pick the one that fits YOU the best.