All of the footage of Dynamaxing we have seen so far shows each Pokemon growing to roughly the same size. This means is increase will not scale for each Pokemon, instead each Pokemon will grow to the same set size.
I've estimated this height using 5 feet 6 inches as an average height for protagonists.
The Grookey in this image is roughly 12 "characters" tall, giving it a height of about 66 feet (yes, this number is an estimate due to perspective differences).
Wailord is 47.5 feet tall
Joltik is 0.3 feet tall

Dynamax Wailord will grow to 133% of its size
Dynamax Joltik will grow to 22,000% of its size