So with trial and error, looks like the hole size for the grommet at the firewall should be 7/16. I drilled 3/8 first and then tried feeding the wire through the hole. The feed wire from the switch to the pump goes through the firewall from the inside first as the largest part of the grommet is on the inside.
What I found was the flat connector that connects at the pump and has to feed through the firewall from the inside is 3/8 inch and will not slip past a 3/8 inch hole (it is hard rubber and does not flex). If you drill a 1/2 inch hole, the flat connector to the pump goes through, the grommet fits too but is a little loose (have to use some strip caulk on inside flat part of the grommet to secure). So using a 7/16th hole would be the right size, large enough to allow the flat connector to the pump to feed through the hole but small enough for the rubber grommet to be able to "grab".
Chris