Hi! The length parameter of beams is indeed not available in the Element ID Manager. The reason for that is that it could cover many different values, it could be the center line or the reference line, if the length is cropped, it could be the cropped or uncropped length, etc.
We would have to make available a whole plethora of values which would be a major development and relatively rarely needed.
However, there is a workaround.
Create a schedule of the beams and click the merge uniform items checkbox. When this is done, you can manually edit the ID-s effectively, with just typing ID-s for the (merged) uniform length beams into the schedule.
best, kata