M6 male terminal is more than plenty If your in anyway worried you can put a small nut on first, then a bigger washer, then your cable, then a washer then the top nut But you won't need to.

If they go flat they can be trickle charged with low amps with no loss of performance. What they dont like when flat is a heavy or boost charge, and thats where people make mistakes with them.

When not in use I have one of those milliamp solar panel chargers connected and it keeps it in tip top condition.