This question has been bothering me for a while and I looked it up using google. The only related search result was this thread http://www.homebrewtalk.com/f11/how-big-your-mash-tun-needs-123585/ which, besides stating the obvious (mash tun size is a function of both the batch size you wish to brew as well as the gravities you wish to achieve), used to feature a chart using which you could find the ideal size of your own personal mash tun. Does anyone have this chart, know where one can find it or have some equivalent solution to these calculations?
I went to the thread and the chart is there. Are you referring to a different chart?
If you make 5 gallon batches typically under 1.060 OG you mash tun only needs to be a little over 4.5 gallons in size. I got to this figure assuming 12lbs 2-row at 70% efficiency to get 6 gallons of 1.060 wort (preboil).
If you then assume that you may on occasion want to make 10 gallons of your favorite recipe you'll need a mash tun closer to 10 gallons (40 quarts). If you have a 10 gallon mash tun, then you'll be able to mash ANY graivity you want for 5 gallon batches.
I remember the chart you are talking about, but I don't think it necessary. You can do the math yourself, however I think that a 40quart mash tun fits most people just fine. I have a 50quart "cube" cooler for a tun and it works great for almost all volumes of mashing I do.