In the model derivatives of Autodesk Platform Service (Forge), after the translation process we can download properties.json. In that file, we have an attribute called "externalId". So, I want to know the maximum length allowed for that field
After downloading properties.json for different CAD files, I noticed that the values for externalId does not have a unique pattern
The External id from Viewer depends on the input file that generates it.
From Revit perspective, it will always be the unique id, which is a guid as described at https://aps.autodesk.com/blog/get-dbid-externalid.
For AutoCAD, it will be the handle, as described at Autodesk Forge - DGN file element externalId
Check aditional details here
There is no documented maximum size for the external_id field.