48 Quart Mash tun

The only issue with a bigger mash tun is it may be more difficult to keep the temperature constant. Throw in some hot water for about 10 minutes before you start mashing to "prime" the mash tun.

I use a 60 qt cooler as a mash tun and I haven't had a problem yet.

 

I use a 52 quart mash tun (coleman xtreme) and have no problems with 5 gallon batches. My brewing spreadsheet does account for mash tun heat absorption, which is reflected in my strike water temp. It holds mash temps very well.

 

I have done grain bills as small as 7lbs and as large as 25 lbs in my 48qt. Putting a layer of foil over the top of the mash before you shut the tun will help hold temp too.

 

Will you be batch or fly sparging?

 

Grain bed depth is of no concern when batch sparging, when with fly it does matter. Make sure the plastic is temperature rated (IIRC CPVC is, but PVC isn't). If you are batch sparging it is MUCH easier to just use a braided hose. I just switched from a copper manifold and will NEVER go back. Clearer runnings almost instantly, and almost impossible to dislodge during the mash.

Also just to add, I use a 12 gallon tun for 5 - 10 gallon batches and love it. The Coleman Extreme may lose 2 degrees during the mash (while opening & stirring mainly I suspect), and I have made everything from 3% Milds to a 13% RIS. It is the perfect size IMHO, as I can jam enough grain in for a partigyle too.

 

Like kjyost said, if you're batch sparging, braided hose is much easier than building a manifold.