It's not the size, it's how long you want to work for (ie how much water you're going to pump during the day)
On your pump it will tell you the amperage (usually about 7). A hundred a/h battery will give a hundred amps for one hour, or one amp for a hundred hours, or any combination in between.
You can work out what size you will need to keep your pump running through the day.
A 7 amp pump will run for 14 hours on a hundred a/h battery
That's assuming everything is perfect. Allow about 20% for power losses, etc, the pump should still run for about 11 hours