Nothing is standardised, the design you posted is Chinese and Chinese swim briefs are 2 sizes or so smaller than you'd expect against Euro or US sizing.
Slazenger are well enough made but nothing exciting.
I can actually make swimwear, I scaled up a pair of cycle shorts for a big friend, he loved them but wanted even bigger so it is very hit or miss getting something that size right first time.
Possibly the easiest swim suit to scale up might be a string bikini, the front and back hang from a waist elastic and meet under the crotch. Scale up the waist elastic to 5XL then ask how wide you'd like the front and the seat to be. The basic design would look somethng like this:
https://www.ebay.co.uk/itm/Men-Thin-...aLK9 v5HVT6vg
The width of the string waistband (elastic in a Lycra sheath) can be adjusted from 1/2" to 2" or more depending on the width of elastic is used.
Don't expect perfection first time, these are unknown waters.