Sharks & rays are generally considered to be for expert saltwater aquarists - because most beginner or intermediate aquarists have difficulties properly caring for them. In other words they tend to make a lot of beginner mistakes. In fact many local fish shops and saltwater aquarists tend to avoid keeping sharks or rays - because they falsely believe that they are nearly impossible to keep in home aquaria. Which is total false. It's just that keeping sharks or rays - require a lot of work, and may be even some thinking out side they typical saltwater box.
Sharks & rays are generally very sensitive to poor water conditions(high nitrates, any nitrites or ammonia), & some kinds of meds (specifically copper-based). For this reason most shark & ray aquarists - have a powerful, effective filtration system for shark/ray tanks and ponds. Also some other fishes(specifically triggers, puffers, angels, lionfishes, butterflies) can be a threat or danger to benthic sharks & stingrays.
As for tank size - that sharks and rays require - that depends on the species. The best sharks for an aquarists looking to keep them - are the benthic species - such as the bamboo sharks,& epaulette(long tailed carpet sharks), and tropical catshark (genus Atelomycterus). These tend to be the most affordable, the most hardy, and easiest to care for.
The Coral Catshark & Australian Marbled catshark (genus Atelomycterus) tend to max out at about 24-27 inches (60-70 cm), As such it is possible to keep one(for life) in a 180-300 gallon tank or pond.
The bamboo sharks & Epaulettes tend to max out at between 30-48 inches(76-122 cm). To keep one for life - you will need a 300-750 gallon tank or pond.
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