Scientists writing in the journal Proceedings of the National Academy of Sciences tracked size change by generations in various land and marine species since the age of dinosaurs 70 million years ago.
"We can show that it took at least 24 million generations to make the proverbial mouse-to-elephant size change -- a massive change, but also a very long time," study leader Dr Alistair Evans from Monash University in Melbourne told Britain's Daily Telegraph.
Changes in whale size occurred almost twice as fast as land mammals in terms of number of generations, the researchers found.
"This is probably because it's easier to be big in water," co-author Erich Fitzgerald from Australia's Museum Victoria said.
The opposite, decrease in body size, can happen much faster, in as little as 100,000 generations, the scientists said.
Many examples of reduction in size have been found on small islands, they said, where limited resources are known to promote evolutionary dwarfism.
"When you do get smaller, you need less food and can reproduce faster, which are real advantages on small islands," Evans said.