Try the steps as shown below:
>> Activate the beam's parent assembly;
>> RMB on this beam and select Find in Content Center;
>> Now this beam should be located in CC and you can change its length;
>> Select Replace All or Replace;
>> You may need to save one instance of this length;
>> Then this part should be updated with modified length and other constraints
and properties should be kept as well. The drawing will update too.