HiXponti
You have to be careful looking at size guides from overseas websites. It's quite common for a lot of overseas kitesurfing locations to get light winds i.e. 12-18 knots, hence the recommendations for larger kites and boards.
One thing that you likely didn't learn in your lessons is that when double the wind speed, you quadruple the power (lift) in the kite. So at around 25 knots, you really don't need or want a very large board as you have 4x the power you would have in 12 knots.
Knowing you are based in Perth and intend to kite in at least 20 knots, a 140cm board would be fine to start with @93kg. You could go as high as a 145cm board but I would not recommend any bigger. if you chose a 145cm board for example, this would be helpful in the very early stages of your progression (learning to get up and ride over longer distances) and also helpful for the lighter wind days i.e. 18 knots or so. Once you are able to ride upwind well, a larger board can be harder to ride and manoeuvre, especially in the chop. So, in strong winds, even at 93kg, a smaller board is much more enjoyable.
You will find size guides for Perth beginner riders on our website here:
www.kitebud.com.au/product/airush-lithium-progression-package-deal-2021/I hope this helps
Christian - KiteBud