Over here, where the growing season is longer, it can get that wider still if left unpruned. A specimen that I was growing, Alice reached 4x4 by year 3 (or 4) or so in my garden. Then we had several months of rain and it caught root rot. In your zone, Alice would grow slower because your growing season is shorter. I would therefore estimate a size of 10' would occur in maybe 10+ years if you do not prune at all. However, its eventual size is a function of many factors (sun light, weather, moisture, soil drainage, fertilizers, competition for water/food, etc) so take that estimate with a huge grain of salt.

I wonder- my Alice was 4x4 it's first spring (planted the previous fall) and would easily go 8x8 here two years later if I did not prune it after it flowered.
It does get regular food and water and plenty of sun.

Thanks for your responses! I think I'll be optimistic and just put in one in the hopes it gets big enough to eventually fill the space. It seems like it would be better to add additional plants later if it doesn't get big rather than try and transplant something that is getting crowded out.

Given the choice between more sun or more shade which would be better? The planting bed gets a range - the left end is morning sun and dappled shade the rest of the day and the right end gets sun all day. Right now the bed is empty (now that I've dug out a 4 inch layer of landscaping rock - ugh) so I can plant the hydrangea anywhere along it's length.

I can't wait to start planting after all the weekends of preparation!

Given the choice between more sun or more shade which would be better?

Morning sun and afternoon shade would be best. If it has enough moisture, it can handle any amount of sun. Oakleafs don't wilt like macs do.

You won't need to buy additional plants. Don't snip the flowerheads after they bloom and let them stay in winter, and you will have a bunch of little babies underneath next summer. Often, a single oakleaf that looks huge is actually a kind of grove from all the self seeded plants. As for your light situation, I have them in afternoon shade, morning sun, and very shaded. They all are the same size and healthy.

Yes, like everyone said, they get huge. When I worked at the garden center, knowing how difficult it is for people to plant immature plants far enough apart, I used to recommend planting the oak leaf hydrangeas on the outside corner of the house. That way, it could go about its businnes getting enormous and just sort of wrap around the corner of the house. And they make a nice corner anchor plant if you don'[t mind deciduous.
Christine

Grew oakleafs in my old garden for 25 years and never once found any seedlings. Never found any seedlings of any kind of hydrangeas at all so I'm not sure how prevalent self-seeding with these shrubs is. At least in my climate......:-)