Reservoir size for flood and drain

Good day peeps.I am switching from soil to hydro and am looking for alittle bit of advice.What is the ideal size res that I should be running on a 4 x 8 flood and drain table ? I have read that 100-120 gallons is ideal but hoping for some feedback.Thanks in advance.

You definitely need enough that your pump stays submerged when the tray is flooded. Keeping the reservoir below the height of the table means it must be low and wide. Usually fit under the tray itself. 100 - 120 gallons is a good place to be, I run my F&D res for a whole flowering cycle with only top offs and add backs.

A good way to make your own res is to use 2x12 lumber and lag bolts to make a "sand box" then line it with 45 mil pond liner. Put a few cross braces on top to set your tray on and you got large capacity and low profile.