A sprint is an iteration and at its end user value should have been created. It includes the sprint planning at the beginning and at its end the sprint review and retrospective. The sprint includes short daily sync meetings called daily scrum or stand-up.
The software engineering perspective
The sprint length can be adapted. Looking at it from a software engineering perspective a sprint takes usually up to one month. Longer iterations increase the risk of changed requirements.
Considering team size
Taking into account what I heard and read from other companies but also judging based on my own experience a team size of around eight seems to be working quiet well. Communication is rather easy. A drop out can be compensated and events have an acceptable length. Usually a sprint length of two weeks gives enough time to build a small increment end-to-end.
Working in a smaller team requires faster adaption as the team is more fragile. A drop out has higher impact.
Considering type of work
Finally, what sprint length works well for you also depends on the type of work. Working in an environment with fast changing requirements a long sprint length - and therefore long-term planning - makes no sense. In an environment with few changing requirements longer sprint lengths are possible and hence long-term predictability is increased.
The family perspective
As mentioned earlier in my family we work with a sprint length of one week. We are a small team - only two if you do not count the dog - and we have a rather different use case for the sprint.
We do not want to optimise for changing requirements or output. We wanted to optimise for better and more frequent communication. A sprint length of one week enables us to see and remember mistakes, misbehaviour but also good teamwork better.
As with almost everything in scrum it is important to keep in mind that the sprint length is something that should be kept flexible. The right length depends on so many variables. Start with some length and adapt if necessary. This also means that a sprint length that once worked for you in the past might not be suitable anymore because of a changed environment.