Hole size and soil questions

I've got a spot where I want to transplant some plants, and i wanted to know how wide and how deep i need to make the holes to plant them in. I heard somewhere that a good 6' diameter hole 3' deep would be good, but i think 6' is much too wide and 3' a bit too shallow. I plan on filling the holes with foxfarm ocean forest potting or happy frog all purpose and then transplanting the plants and then putting mulch on top of that. Your thoughts and suggestions would be appreciated :]

i think 6ft is too wide and 3ft is plenty deep. i think a 2x2x2 hole is good but i guess it depends on how many plants your growing. if your only growing 10 plants, why not dig big holes. maybe 3x3x3. one thing is for sure, your hard work will be rewarded.