1 lb of 2-row has a potential for 36 points in one gallon.
42 lbs = 42 * 36 = 1512
At 50% efficiency that's 756 points
In 5 gallons that would be 756/5 = 1.152
To get 1.080 post boil you need 756/x = 1.080. Solving, x= 9.45 gallons.
I'm assuming you want 1.080 post boil. So, now you need to factor in loss to the break and evaporation during the boil. But you'll need to start around 13 or 14 gallons pre-boil. Maybe more if you boil 90 minutes.
So, you pot should be at least 15 gallons.
But do check my math And you can figure that for 132 lb of grain, you can scale accordingly. Your kettle will need to be almost the size of your mash tun...