Hello François GUENIOT

You need to use Custom CSS for this. For example, please use this CSS to Template Options -> Custom Code -> Custom CSS

.sppb-addon-table-main tr td:nth-child(even){
  width: 100px;
}

This will set the width 100px of all even columns. For odd columns, use odd instead of even. You can also use it for any specific columns by calling the column number.

Best regards

You are welcome.