Reserviors in a hydroponics system should be large enought to accommodate the needs of the mature plants. You can calculate the approximate needs by counting the number of plants and their mature size. Large plants, like tomatoes, will need about 3 gallons of water each, medium size plants like spinach and other salad greens typically require about 1.5 gallons, and small plants like herbs need only about 0.5 gallons. These are minimum amounts, so it's wise to increase (even double) the volume of your reservoir so that you're not caught in a situation where there's too little water due to evaporation or heat stress and and your plants suffer.
Time spent in a reservoir is important since that's where you can actively improve the quality of your water. This is the step where testing the water for nutrient levels, adjusting levels by either adding more nutrients or diluting with fresh water, happens. This is also where aeration can be used to increase the levels of dissolved oxygen, which is best taken up by the roots, and which discourages growth of anaerobic bacteria and problematic pathogens. Reservoirs are important as holding tanks, too, where water temperature can be adjusted as necessary.
