If you have made the modern (and sensible/efficient) choice of a submersible pump, it's mostly just choosing the right one for the well you have. You generally won't use the same pump for a 75 foot well and a 300 foot well. When shopping you see what's available that includes 6GPM in the 300 foot depth box if that's what you are looking for.
I suppose you can use PVC-80 sticks - I personally have a pump hanging on 200PSI black polyethylene, 300 feet down (289 feet of pipe in the well, due to where the pitless is on the casing, and how long the pump is.)
Wire just needs to meet the requirements of your pump, and there are detailed charts in the pump manual. Total distance matters, not just the down-hole portion of it. IIRC, 2-wire (+ ground) allowed smaller wire than 3-wire (+ground) for my pump. I use 2-wire for the same reason I don't use a deep well jet - the reliability of the electric parts has come such a long way that there's really no need to keep them on the surface, and the 2-wire submersible is more efficient.
If you have 3-phase available and a need for that much pump that's a different ball of wax, but 300 feet and 6 GPM should not require that.
Pipe size is a function of total pipe length and flow rate .vs. friction head and velocity in the pipe all balanced against incremental cost. For 6 GPM 1 inch 400 feet of (plastic) pipe (if its 100 feet or less from the well to the house) is under 5 PSI friction head at 6 GPM at a velocity of 2.45 ft per second. Which makes the increased cost of 1-1/4" pipe a dubious place to put money.