I spent far too long trying to remember my GCSE geometry in an attempt to work out how long the sides of the square needed to be to give me a particular measurement diagonally. In the end I gave up, ate some chocolate and emailed G at work, who reminded me that it was Pythagoras that did what I neeeded: A squared plus B squared equals C squared. Helpfully he'd done the actual calculations for me as well, so I didn't have to start wondering whether A, B or C was the diagonal and how the square root formula worked in Excel.
So I cut out a 26" square from a large remnant of Moda fabric I had been hoarding. I hemmed each side with a double quarter inch hem to give me a scarf that measures 25.5" square and a little over 35" on the diagonal.
As I sat down to sew I was listening to Woman's Hour on Radio 4, and in one of those delightfully coincidental moments they had a discussion on headscarves. Perfect!