We usually recommend The Strøm 60L Rollerbag (The Little Bastard 60L) for trips up to 14 days (two weeks) and The Strøm 90L Rollerbag (The Big Bastard 90L)for longer trips. It also depends on what you pack - winter gear usually tends to take up more space, meaning The Strøm 90L Rollerbag (The Big Bastard 90L) could be useful when going on trips with plenty of gear.
Sometimes airlines demand to mark the The Strøm 90L Rollerbag (The Big Bastard 90L) as "oversized baggage" with extra cost, due to its size. We would recommend checking the approved check-in luggage sizes with your airline.