As to how I know that Jackson boots will fit me: I don't. That's why I want to try them on before buying. It's pure guesswork.
Jackson size charts:
Jackson Ultima is a global leader in figure skate boots and blades. Our expertise and passion for the sport is embodied in Jackson Ultima’s commitment to technology that enhances the performance of the World’s leading skaters. We also offer a complete range of recreational figure and leisure skates.
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ic3Rabit seems to think Jackson & Risport are the only two brands of stock skate suitable for people with wide toes. I don't know anything about Risport, other than that the (relatively low level - single leather thickness) almost new boots I bought used some years ago broke down in a month.
Though a skating lady of my acquaintance just pointed out that it might cost less to buy used boots online than to travel hundreds of miles to a shop that has them in stock, if that even exists. If they fit, but are too broken down, I could go to a shop to order new ones. And if they aren't too broken down for me to use, I've saved my self a lot of money. If they almost fit, maybe I could order Jackson Rapid Custom boots. Since I've already done things like heat mold & punch my own boots, mount my own blades, and make my own insoles, I can do myself much of what a good skate tech would do for me, which makes buying online a little less dangerous for me than for most skaters. OTOH, I may not be able to find 6 wide - maybe just 6. If the used skates don't work, maybe I can resell them - though that might be hard for the same reason that finding boots in my size is hard - not many men have my shape & size feet.
Jackson Freestyles might be too stiff for me. I'm only doing 1/2 rotation jumps, and am getting older. But they don't list anything suitable that can handle more than Basic 6 (which only has bunny hops).
By those charts I'm a Mens 6 M (medium), maybe a little less than M. But the widest part of my foot is the toes - and most boots narrow at the ends. Also, my toes & metatarsals are a bit on the small size, including in height and the width measurement is a circumference (distance around the foot), not a true foot width. Plus on almost any shoe, width is the limiting factor for me - I usually have to buy 1 or 2 sizes longer than I otherwise would. (Womens sizes are narrower - as shown by that chart. They also narrow more at the toes.) So I'm going wide.
I'd rather have a little extra space to play with that I can fill, than not enough space - especially in the toe box, which is very hard to stretch. For people who don't know how to fill space, by making their own insoles, that might be a mistake.
I rather full customs again, but measured right this time, but the prices and production delays are so very high. (Harlick is quoting two years and $1795, Avanta is about 1/2 year and $2250. Jackson "full custom" boots don't allow much customization along the lines I need. My boots are already so soft it hurts to jump at all.)