Acer palmatum ‘Geisha Gone Wild’

This is my first choice, but only by a hair. It is a rare and unusual maple and exactly the right size when mature — six to eight feet tall by three to four feet wide. It would never outgrow the space. You can also see more pictures and read more about the variety here. The pictures on that site show a very rich pink which I do not think you would get in your light exposure. Not sure if grower has available but can check.

Photos on this site show bright pink foliage colour early in the season and then a green edged with white later in the season which is what I think you would have most of the year, with a less intense pink than in the pictures (but still pink) in the spring when it leafs out.

Acer palmatum ‘Koto No Ito’

You can see a range of pictures of this stunning Japanese maple and the foliage colours at different times of the year here. You can also read about it here.