Well. first off I am lazy, and feel that all this squeezing and sqooshing the grain bag for a half pint of wort is rather silly...sorry JMO. I simply hang the bag from a hook in the cieling and let it drain. I have never used a built in thermometer, but I have read that it does not really present problems if a little bit of care is used while removing the bag...granted one does not snag the thermo and just keep pulling. Just push the bag away from the thermo prior to removal. I'm a fan of simple hand held thermos, once you hit mash temp, leave it alone IMHO. It is also nice to check the mash in several spots...a mounted thermo doesn't do this.
How big a pot...well a 10 gal will mash 15 lbs at 2.2 qts resulting in a mash volume of 9.45 gal. So that is 33 qts total water or 8.25 gal....if the grain absorbs 1.25 gal, boiloff is 1 gallon and trub loss is a half gallon, your left with 5.5 gallons. So w/ a ten gallon kettle mashing full volume you are maxed out at approx. 15 lb grain bills...but w/ a dunk sparge, you could go more??? So ten gal is a good size IMO considering you could occasionally dunk sparge on silly big huge beers.
ref: can I mash it...below