* Your van's door jamb sticker: This sticker often lists the original tire size and other important information.
* Your owner's manual: If you have the original owner's manual, it will specify the recommended tire size.
Without this information, it's impossible to give you a definitive answer. Common sizes for that era of van *might* include options like 7.75-15 or 8.9-15, but these are not guaranteed to be correct for your particular G30.