A: It is 3 inches in diameter for the tip.
A: You will need steel drive pipe to continue to drive the point down. If you are already hitting water, then no need to go down too much further. But you won’t really know if you’re deep enough until you connect a pump and see if you get a consistent flow and see that the water replenishes at a rate to keep up with your pump and water usage. Assuming you are using 2” for irrigation purposes, yes, you can connect that to the pump, but may reduce down of your pump inlet is smaller. You will also want a check valve connected close to the pump, and possibly a sediment filter to keep out sand from your system.
A: General recommendation is for the well point to be several feet down AFTER hitting water to assure you of a positive water source.