Does size matter?

Is it weird that I am really excited to have a new suitcase for our Italy trip?  Jeremy doubts that I can fit everything in it that I need but I am determined to pack light.  We are still debating what size/type of suitcase is appropriate for travel in Europe.  I have heard to keep things small and light and I am willing to do laundry if necessary.  I also think that the high temperatures will mean I don't need any heavy clothing so that should help!

If you've been to Europe, I'd love your input into the luggage debate.  What size/type of suitcase did you take and what kind of day pack did you have along for touring?

I bought a 21" Hey's Velocity hard cover suitcase with spinner wheels and it's also slightly expandable width-wise.  It's super small compared to my usual suitcases for traveling but I think it'll be best for train travel and hauling around Italian cities if we need to walk a few blocks from time to time.  We aren't leaving until August so I have lots of time to test it out to make sure it fits everything I need.

So...what do you think?  Does size matter?