LOL!! They come a lot smaller than that! I'd be surprised to see 'Sharp's Pygmy', 'Hupp's Dwarf' or 'Yuri Hime' hit much beyond 3-4' and 'Mikawa Yatsubusa' very much more than that. And there's quite a few other cultivars in the same height range. Many of these true dwarfs (Group 6 in JM classification) are used widely for bonsai.

'Red Pygmy' is actually considered a semi-dwarf in the Group 5 classification, Linearlobums, and could get as large as 10-13' with a spread of about half that. Growth rate is slow to moderate.

Ultimate size of any cultivar of Japanese maple is always a matter of some discussion - check 3 different sources and you'll get three different answers :-) It may be best to plan for the largest size you find listed but expect the smaller.

The garden center I bought my maple from had a few 6 feet tall Red Pygmy that are about that wide as well. They certainly don't look like 20 year old trees. I wonder if they are some other linearlobums masquerading as 'Red Pygmy'.