To save you the google search, it's about "tennis ball size" according to https://30a.com/larger-pacific-striped-octopus/ . Somebody should add that to the wiki.
"The most obvious differences between different animals are differences of size, but for some reason the zoologists have paid singularly little attention to them. In a large textbook of zoology before me I find no indication that the eagle is larger than the sparrow, or the hippopotamus bigger than the hare, though some grudging admissions are made in the case of the mouse and the whale."
> "The LPSO is a small animal, with a striped body about 7 centimetres long and spotted arms that make it look like it is wearing a polka-dot dress."
Anytime you're looking for more info on topic while reading its wikipedia article checking the sources is a good place to start.