For standard LabKey Server grids, you can customize column widths via .query.xml metadata.
When you set a width for a column that's a lookup, the value is ignored. That's because we're considering the lookup target's title column as the displayColumn and pulling the width from it.
https://github.com/LabKey/platform/blob/386e084f449e6865ec65a8e5254743f8e8d0acc3/api/src/org/labkey/api/data/DataColumn.java#L96
This seems like reasonable behavior, as in many cases you'd want to set this centrally instead of every place that uses the lookup.
However, it also seems reasonable to allow this to be customized on a case-by-case value. I propose that we use any width setting on the field that's doing the lookup by default and fall back to the lookup target column when we don't have a width set (which will be the vast majority of the time).