According to the IVA manual I need to earth my fuel tank, but it makes no reference to the size of the earth wire/strap?

It would be really convenient for me to mount it to the bolts for the fuel pump but it would have to be a relatively small wire, is that ok? Is there any particular minimum size I should be using?

Thanks

Any size would do. I suspect it's just to guard against static build up/discharge at the wrong moment.
Is it not bolted to the chasis anyway?

find a dumped fridge or microwave, chop off the mains cable and use the earth wire - plenty big enough.

You'll need to ground the sender plate for the gauge to work but that may well not connect to the tank itself.
John.