If it’s the first column, you can put
table#wpt_table.wpt_product_table th:first-of-type {
width: 150px;
}
For other columns, you can use
table#wpt_table.wpt_product_table th:nth-of-type(5) {
width: 250px;
}
Change the 5 to whatever number column you are trying to affect. Also, obviously, change the width to whatever you need.