I've yet to see any refugium/sump I'd describe as overkill. Keep in mind, the volume of water over a coral reef is typically replaced completely around 2-6 times per day. The space available for nutrient processing is vastly greater than we have in captivity (hence we often need equipment to make up the difference). You'd probably eventually get to a point of diminishing returns with a sump scaled larger and larger, but bigger is better IMHO. If feasible I would keep a sump/refugium several times larger than the main display. The higher the water capacity and nutrient processing capacity relative to the biomass of the tank, the fewer the problems that will crop up, and the lower the chances of disasters (i.e., tank crashes). A 75 gal sump/fuge with a 125 gal tank is perfectly functional, and I certainly wouldn't go smaller unless absolutely required.
cj