Hi Ruth, I come from a long line of generously sized noggins. I normally block too small blocks with wool felt first, I allow this felt to dry and then I block my final material on it. Multiple pre blockings may be required to get the size you need, you just have to be careful elaborately designed blocks that you don't lose the details. I don't care much for hat stretchers but if it only needs to be enlarged the smallest amount if need be before the shape gets distorted.
Hope this helps you in some way. :)
Thanks Megan, I had thought of using an old felt hat to cover it which would bring it up to the right size, but I am concerned about how to fix it onto the block and still be able to remove it if necessary. What would the fixing method be?
Ruth just use strips of old newspaper which you wet and wrap evenly round the existing block of choice - use you tape measure to check and verify that it comes up to the right head size then WRAP adapted block with cling film 2 to 3 times and bob's your uncle.
I had a situation similar to yours where a client of mine with a 30inch head and not much money needed a school hat made for them and it worked just fine.