The calcuation for the DATA_LENGTH field of the _USER_COLUMNS system table is not documented; however the calculation is referenced in the following Technical Tip: 4D Data Types in SQL System Tables.

The 4D Data Types in SQL System Tables Technical Tip was written for v11.1 for which the information is accurate. In fact, the information is also accurate in v11.0 through v11.9 and v12 up through v12.4 as well as for v13 through v13.1. However the DATA_LENGTH calculation of Alpha fields changed in v12.5 and v13.1HF1; in those versions (and the versions since then) the calculation is now alpha_field_length*2.

This is important to know if you have code that uses this value, such as the Compacting Address Tables Technical Note.