1” is more than sufficient if you plumb it correctly. I get 7+ gpm of gravity flow to my pump with my tank only half full.
Not sure, but I think @Innocentbystander might even plumb his 8gpm machines with 1” crushproof line.
Plumb in a 2” bulkhead for your dump valve, though. No need to wait around emptying your tank at 7gpm.
