At our coursing club we had no mains water so we pumped the water out of the ground into a holding tank on stilts which from memory was 3000 gals but that's a huge expense, If its just for your own use I would think about a towed water bowser, put a tow hitch on one of your vehicles and you're away.

Is your tank getting refilled from a tank pump or is the water from a mains water line or from only catchment rain water ....
Check your water PH to ensure its sits within range as you will get a lot better grass growth .

What kind of soil and grass are you trying to grow in

How often do you mow it ,
How long is the straight track as you will need to split the line if its over 200 meters so that you get enough water further up the line ..

To answer your Question
Bigger is better get a huge tank ...cheers