If I may Chris,
I believe the question is to rather the length is 'straight' or 'follows the arc'. In this particular case I don't believe there will be a significant difference.
In the majority of cases you would want to reference the drawings/details and see if you are talking center line of beam following the 'arc' and end to end OR if you are talking about the 'chord' which would be straight across regardless of how long the beam itself was from end to end.
The only reason to really consider this is to make sure your curved/radius beam is long enough to do the job BEFORE you set the camber/radius to it.
Have a Great Day, Brent