a reservoir of any size (imo) in RDWC, would just require it to be at the correct height. It's all about gravity and moving the water that you pump in causing recirculation.
For example, 4 x 5 gallon buckets, I'd think normally I would run something like 5-10 gallon reservoir that fills; if you wanted bigger, maybe a full tote or something? that would work I think. You just don't want the reservoir to be too tall. example of too tall would be like a 65 gallon drum. You'd have to test it at different levels of height right? Most RDWC is gravity flow, as in water fills the res, gravity or weight of the water at a higher level starts to recirculate. If your reservoir height is far above the level of the buckets, it will just over fill and spill out ya know what i mean? What are you working with for the 65 gallon res?