1. Check the sticker on the driver's side doorjamb: This sticker will list the manufacturer's recommended tire size and inflation pressure. This is the most reliable source.
2. Check your owner's manual: The owner's manual will also specify the recommended tire size(s).
3. Look at the sidewall of your existing tires: The size is printed on the sidewall of your current tires. However, this only tells you what's currently on the van, not necessarily what's *recommended*. Someone may have put on the wrong size previously.
Without access to your specific van's information, I cannot give you a definitive answer. Common sizes for G20s were in the range of LT235/85R16 or similar, but there could be variations. Always prioritize the information found on the doorjamb sticker and owner's manual.
