Door Flush Bolt length ?

[VapourSnow]

For a new pair of internal wooden doors 686 x 1981 We want a flush bolt fitting to hold one door closed. The flush bolt will be morticed into the edge of the leaf.
As flush bolts come in different lengths, how do you choose which length please ?

 

[opps]

I guess it depends on the height of the person that will be using it.

 

[foxhole]

Just use shortest for domestic door .

 

[JohnD]

Though a longer one is easier to reach.