I suppose it'd be a trade-off: smaller bag might save you a checked luggage fee on an intra-Europe flight, but it requires more frequent washing of clothes, however you end up doing it.

Considering most European airlines will give you a 20kg allowance on short-hauls, that would likely be the happy medium I'd aim for. Well, somewhat below that, in case I acquired things while I was there.

I just bought a few intra-Europe tickets for some summer 2013 travel...the so-called "low cost" options were mostly using strange airports that would have generated a far higher total (time + cash) cost overall. Going in, I thought maybe I'd be buying these 1-euro base fare seats and paying all sorts of extra fees, but in the end I'm just on airlines like EI, AF, and LH for 80-90 euros per segment, one bag included, but flying to/from the exact cities I'll actually be in.