Tianhuangping Pumped Storage Power Station has a 13GW/h storage capacity and about 2GW generating capacity, using two 6.8 million m^3 reservoirs - one artificial, and one built on the Daxi Creek, separated by about 900m of pipes (but I believe the vertical drop is about 600m). (if you're curious, that adds up to about 1/7500th of the worldwide average usage in 2008)
If you assume 50m high structures (the lower dam is actually 72m), the area required for each reservoir is "only" 136,000 m^2. With only 20m tall structures, you need about 340,000 m^2
That's within range of the largest buildings (e.g. Boeing's largest factory has a surface area of about 400,000 m^2, and a volume of 13,3 million m^3; Tesla's factory covers an area of about 510,000 m^2).
Of course this is assuming you can find somewhere suitable to give you a 600m drop, or water volume required grows quickly. But the system could be mostly closed to reduce the need to be straight on a major river.
And in Japan there's pumped storage plant with a 250,000m^3 pool 500m below ground, so clearly it does not need to be a matter of building one of the reservoirs 600m up a mountain side like with Tianhuangping.
To the extent the ecological risk is massive, it's because it's presumably far cheaper to pick a suitable river and dam it than having to build two fully artificial reservoirs.