A good rule of thumb is 12" of height/per gallon and to double the size of your existing pot on transplanting. Other factors involved in determining pot size are your grow room footprint size, the amount/intensity of light plants receive, where plants are in the grow cycle and if the are from seed or clones. Grow room area size: If your grow room foot print size is small and horizontal space is at a minimum then BOG's (Bushy Older Grower) doubling potting method is a good alternative. Rather than repotting into a larger diameter pot your existing pot is set on top of another pot a few inches more in diameter. Thus giving your plants a lot more vertical root volume of soil to grow into and less horizontal volume. Amount of light and intensity: Plants that are under low wattage lights (70-250) tend not to grow as big and as quick as plants under high wattage lights (400-100), thus one can scale back pot size slightly. Also if your plants are under a 24/0 cycle or an 18/6 cycle this will affect growth rates and repotting sizes. Growth cycle: The average grower will find that they must repot usually every 2 weeks while in the veg cycle. Going from a 1-1.5 gallon to a 2-3 gallon in the third/fourth week of the veg cycle. From a 2-3 gallon pot to a 4-5 gallon in the 5-6 week of the veg cycle. Note it is always best to repot 1-2 weeks before you flip your lights to the 12/12 flowering cycle and to let your plants finish off in that last transplanted pot size. Seed or clones: This will make the biggest difference between pot sizes. Since one must veg up to 4-6 week from seed to reach plant maturity. Cloned plants need not be veg as long 1-3 weeks. This will drastically reduced the pot size needed for a cloned plant. These guide lines are by no means written in stone, all environments are different and will produce different growth/repotting rates this is just a basic outline.