Is the Configuration Specific Property "Length" in a Part or at the Assembly Level?
You have "Weight" set up and you say that works. Do you have the Part, that "Weight" is in, captured in DriveWorksXpress?
If the Component/Part, with the Length custom property in, is not captured in DriveWorksXpress then it will not do anything with that file. It will not remove anything from that file either. Do you have that Part Captured in DriveWorksXpress?
The other thing to check is that DriveWorks is actually using the part you think it is using. Sometimes, when capturing and using multiple parts, the SolidWorks references can get confused. In the Generated file, or in the captured assembly, check the Model references and make sure it is using the file you want it to use.
It should not matter that it was a custom property that you have added.
Do all of the calculated lengths in the BOM come out at the correct values?