I am always amazed by looking at people with hugely oversized suitcases at the air port. And then they wrap them.
But now that I need a new suitcases (my old one has a taped handle and broken wheels), size no longer is a question of comedy.
My old one has a size of 60 x 39 x 29 cm.
My parents have 66 x 42 x 24 (height including rolls)
Maybe it is not a question of size but of volume? These seem all close to 60 L.
Another thing to keep in mind is the maximum allowed size of the airline -- this seems to be 158 cm (L + H + W or W + D + H).
I have two options: 44 + 67 + 28 and 48 + 78 + 31. I choose the smaller one, as I won't have to worry about weight restrictions then as much -- it is difficult to cram too much into a smaller suitcase :)
Upd: chose the bigger one (size L), as it still fits below the 158 cm (48 + 78 + 31 = 157), and I sometimes had the problem of visiting my parents and not being able to fit in everything. Also, the suitcase is just 5 EUR more expensive than the size M one.