Leaving a job after a short time. How much info to share?

[Anonymous]

I plan to resign from a job a few months after starting: it is not a good fit. I don’t want to burn bridges as this is a fairly large firm I may encounter again, so I plan to be very positive and of course give appropriate notice.

I’m sure the team will be wondering why I am resigning so quickly. I don’t really want to get into it or share where I am going. How much do I realistically need to share other than it is not a good fit?