Wiring size for 75-foot run from 40 amp breakers to tankless water heater?
Put in big conduit.
The actual wire size for your 40A circuit is 8 AWG - see What Wire Should be Used for a 40A 240V Circuit? for details.
However:

You may later upgrade to a bigger heater requiring larger wires.
You may later upgrade to a much bigger heater that requires two circuits and twice as many wires.
You may later decide to run something else to the same location.

Put in 1/2&quot; conduit and you'll be fine now but have very little room to grow.
Put in 3/4&quot; conduit and you'll have some expansion room.
Put in 1&quot; conduit and you'll have room for anything from 2 or 3 large circuits to a single circuit for a big subpanel.
At current pricing, 60 ft. (based on &quot;50 - 55 feet from the panel&quot;, though title says &quot;75&quot;) of 1&quot; Schedule 40 conduit at Home Depot will cost you $13.92 more than 1/2&quot;. Plus a little extra on the fittings. $20 now will potentially save redoing everything in the future.
Don't run any wire in that conduit without checking with your electrician. He may be particular about how to do things - or he may say &quot;Any #8 is fine, leave me a few feet on each end to work with.&quot;