This question comes up a lot in new scrum teams. The adjustment from a couple of months to two weeks is very tough indeed. Generally speaking do not keep changing the sprint length. One Sprint two weeks, the next three weeks etc.
One of the main tenets of Scrum is “Sustainable pace”. Scrum teaams are able to get to this sustainable pace as they get really good at delivering something in a short span of time ie, 2 or three weeks.
Velocity cannot be precicted well if the Sprint length changes. If you think changing 2 to three weeks is going to help you, you are mistaken. The team will only end up signing of for more work than they normally would. Hence the problem has really not gone away.