There are so many variables, it is impossible to directly relate what you read about to what you would actually use.
Level of sailor experience
Fresh or salt water
waves, chop, flat water, etc
Body weight & height of sailor
Type of board (slalom, freeride, freestyle, wave, formula, long board, etc.)
Weight, volume, width of board
Fin type and size
Type of sail (wave, freeride, slalom, formula, freestyle, etc.)
Everything above will have some impact on choosing your sail size. For example, I sail on inland lakes in Texas. It's not uncommon for a bunch of us to be on the water at the same time using sail sizes anywhere between a 5.7 and a 11.0. One of my buddies who is very light weight on very light custom slalom boards would be on his 5.7 in 12 - 15 knots, while I may be out on my formula board with a 11.0. The rest of the gang on average may be on 7.5's to 9.0's.
When I get to the lake and there are others on the water, I ask what they have been using. I also look at the water and chop to estimate what I will use along with the info. from the other sailors. If the wind was 10 to 20 knots (pretty variable, but normal lake sailing conditions), I could choose to sail my formula 160 on a 9.2 & a formula sail; or an iSonic 111 slalom board with a 7.5 race sail.
Two people, one a novice and one an expert, both the same size and weight on the same board and sail may choose sail sizes 2 to 3 meters apart based on their skill level. The more experience you gain, the bigger sails you can manage and contol.
It just takes a while to figure out what works best for you, and even the experts get it wrong from time to time and have to come in to re-rig another sail.
I hope this helps.