For a 'normal' gravity 100L brew you'd need 20-25kg of grain. To mash that in you need 2.5 times as much water (about 50-60L) so in total it should be a 70-80L mash. If you're fly sparging you should just about squeeze it in although there won't be much room to spare.
They are, but you'd need a 120ltr + boiler to boil 90 - 100 ltrs safely.
I use a 100ltr nordic pot for a copper and it's pretty full with 76ltrs in it.
0.5cm = 1ltr in the 100ltr pots so 24ltr spare on a 76ltr brew is only 12cm...not a lot of room for boil over or a vigourous boil :wha: