I got nothin’ for you. sorry

I have had Sango Kaku in almost full sun in zone 7, hot summers. It may be larger than what you are looking for.

What do you consider to be medium sized? JM's are by nature understory trees and would always prefer some protection from hot afternoon sun. But having said that, most red leaved forms are tolerant of very sunny positions - provided they receive plentiful irrigation. And that does not necessarily limit selection as some of the largest and smallest JM's have red leaves.

The top of my short list would be Fireglow.

I have Fireglow. We bought it in 2011 as a bonsai specimen for ten bucks and repotted it. It's 12" tall here.

I am in Portland Oregon zone 8. It's been through extreme heat in June before it could acclimate to sudden temperature swings and the upper leaves did turn crispy. I may have been able to avoid that if I had watered it very well the day before, but we'd never had 100 degrees in June so I didn't know. Now I do water deeply whenever we have 90+ in the forecast.

It's best in spring. It's at its reddest when light shines through the leaves, so you wouldn't want to plant it in full shade. Morning sun is ideal, though mine also gets mid-afternoon sun. The leaves get dull red with tinges of green in summer, probably due to the PM sun. It doesn't have fall color. It does have some branch dieback in winter. It's really not much different than the common Bloodgood, which my neighbor has bordering our front yards.

I use it as a centerpiece in a shade bed.

Thank you for all of the info! What a beautiful picture of your garden! We have several JMs which are doing beautifully, one of which is a bloodgood that started as a twig about 10 years ago and is now at least 15 feet tall and wide. We also have lost several through the years. The spot I am trying to fill had a 4 year old Waterfall JM that seemed to be doing great, but died over this past winter. I would like something that wouldn’t get over about 10 feet, and I would like it to have beautiful gold and orange colors in fall if possible.

I had a generic Acer palmatum I think from the Arbor Day Foundation in a damp full sun spot for a decade. It grew to maybe 8 or 10 feet tall then just up and died probably from verticillium wilt if I remember the thread on here's decision.

This is a weeping Ryusen japanese maple. Full sun. ( dressed for Halloween,)