Bigger the better. If you find you are losing temps during your mash, you can cut out some foam insulation inthe approximate shape and lay over your mash to try to prevent temp loss in the headspace.
The bigger the tun you have, the bigger gravity beers you can do, or the bigger volume of normal-gravity beers you can do.
Read
this thread for a pretty in-depth discussion of tun sizes as related to allowable volumes and gravity numbers. It's very helpful.
@EdWort
This is the first post of yours I've seen since bottling my first attempt of your Haus Pale Ale. The green beer left over after bottling tasted amazing, and I have incredibly high hopes for the finished product. I'm sure you hear it all the time, but thanks for posting such a great recipe.
Cheers!