Dear Wilaiporn,
A) You need edit admin_vehiclemanager.css
/public_html/main/administrator/components/com_vehiclemanager/admin_vehiclemanager.css
we added body.task-edit .add_vehicle .adminform tr td select#catid {
width: 500px;
height: 250px;
}
You can play with that settings. BEFORE MAKE ANY CHANGES, please BACKUP file.
increase input field width of item with different size not the same width of all item fields) not possible.
That fields have no classes, you can not add style for separate field.
Field as you can see consist of tr> and <td>;
<td> and <tr> have no any possibility add own style
Best Wishes,
Taras
OrdaSoft team