Mr. Toad, have you visited the website, "Hydrangeas, Hydrangeas" and read their information? Really interesting website.

Rosie

Here is a link that might be useful: Oakleaf Hydrangea

Larger how? Bigger overall shrub size or flower size? 'Alice' could arguably win the prize for both, although 'Snowflake' could edge it out on flower size (followed closely by 'Harmony' and 'Roanoke').

Dirr describes a planting of 'Roanoke' at Bernheim Arboretum as 10' tall by 30' wide but doesn't indicate if it was a single plant or multiples.