This is an old question:
https://mxforum.mendix.com/questions/1923/Resize-or-rearrange-datagrid-search-fields
About changing the widths of search fields. It gives the answer I think I'm looking for:
.dijitInline mendixDataGrid_searchItem { width:150px; }
However, I don't know where to put this (what I am assuming) is CSS. Any pointers?
Cheers.