A whale has to be defined in relation to the trading environment.
On a small exchange, a person selling 3 BTC of a cryptocurrency can definetly be a whale.
On a huge exchange, it might have to be 300 BTC though...
What makes a whale is the possibillity of obvious price manipulation.
Yeah, liquidity in a market or a particular exchange also matters anyway but except the market is only available on a particular exchange, that is when being a whale on that exchange may work, but at the same time, low volume exchanges can be very hard to penetrate or make any impact as a whale as you will really need a big market, but I feel you though, in a very good trend, it can be an easy thing to do.