Here is a visual comparison of some of the "Big 7s" with the 26Nosler. This information was garnered from Ammoguide.com. They are arranged by H20 capacity posted by Ammoguide.
7mm Rum - 106.6gr.
26Nosler - 96.4 with a 6.5 neck. A 7mm neck would add a few gr.
7mm STW - 92.7gr.
7mm Dakota- 90.7gr.
7mm Mash - 84.9gr.
7LRM - 84.8gr.
The 7Dakota should be after the STW - my mistake
It looks to me like the Nosler case is the largest that will fit in a 3.6" action with available, quality brass. I'm planning on using a RUM action and throat the barrel long for the 7mm heavy bullets. I'm thinking that the "useable" case capacity will be close to the RUM's. Something like a larger 7LRM. I'm also thinking of a 338x26Nosler on M70 3.6 CRF action.
All information was off the "Internet" so take it for what that's worth although I've found Ammoguide to be pretty reliable.
SCP