They can grow much taller in a suitable climate, especially the species form. A number of public garden in my area have JM's that exceed that height - Bloedel Reserve, Kubota Gardens, the Japanese Garden at the Washington Park Arboretum. Many of these are very mature trees that have been in place for decades. But the PNW is pretty much an ideal climate for Japanese maples so they tend to do well here, establish readily and grow large :-))

I have, but it is also rare enough I noticed them. They were apparently popular in the 70's even and are located in old protected courtyards around town.

There are several in my area. One down the street is taller than the ridge line of the one story house so definitely in the 15-20 foot range. Planted in the late 1970's would be my guess.

I have several over 20 ft. tall. They are in the 30 year old range.

Japanese Maples over 20 feet in height? I'd say 50% of the houses around here have bloodgoods well over 20 25 feet, I figured that was like that everwhere.

Yes, Bloodgoods are one of the larger growing forms as are its close relatives, Atropurpureum and Emperor 1. Osakazuki is also quite large when mature.

In Japan, these trees can reach as much as 50' in height.

We have a red japanese maple (I'm guessing a generic) planted in a high school common area. At a recent festival, I saw no less than five small children IN the tree climbing it and standing on the horizontal branches. It's a huge JM probably 30x30'

Are Japanese Maples over 20 feet that rare? I bet there are literally 30-40 of them within a 5 mile radius of me right now, lol. Every other house has one.

I dont see a lot of very large ones around where i live at it central arkansas.

I don't see many here in my 5b zone, either. I think for a long time local nurseries didn't sell them. It's only been in the last 10-15 years that it's common to see them for sale here.

Obviously they can thrive, though. There are a few that are huge.

Funny you should ask that.
I was just going through my list to determine which of mine to put in the ground. Which would be tall and take horrendous full sun at the same time.

My list is
Inazuma (already in the southern MD ground, full sun for 6 years) 25-30'
A.S. Palmatifolium 25-30'
Trompenburg is said to have it 20' but based upon a nursery in my area, they have a 15 year, in the ground, in full sun specimen that is 25' or taller now.
My Vitifolium should hit 20-25'
Red Emperor the same.

These are the ones of mine that I've looked at so far. I'm still researching the area. There are a lot that can be 15-20', but your criteria stated taller.

BTW. I look at nurseries in my climate type. I have an unusual microcosm (according the the National Capitol Parks and Planning) that is similar to Atlanta. What good would it do me to look at growth habits in the PNW, right?

Sango Kaku at a golf course I visit. Around 30 ft. tall.

Geeze, I forgot about my own Sango Kaku! It's looking like that, but not yet THAT tall! :)
Christine