On phone app main Today screen tap days of exercise tile, then gear icon near top right. Stride Length is at top.
I'll just comment if you are doing this for the walking stride length.
You don't want exercise pace stride length - because that is not the majority of your day probably.
Just as your day isn't spent at grocery store shuffle pace either, that would be an extreme the other direction.
Walk for 1 km according to a track or known distance (not the Fitbit), starting and stopping a workout correctly so you can see how many steps in that distance.
Walk a pace midpoint between the extremes - like 2.9 kmh. Oh, it'll seem slow and hard to do - but it allows for better accuracy adjustment up and down for your normal daily paces.
If this is for running - go for it, again suggesting mid-point from jog to sprinting speeds you might normally do.