Part of being an adult is about memorizing things about yourself that you need to write on forms. Or that you need to know when you shop for clothing. Just recently I bought a coat online that was labeled as being a size 38R. I thought, "Is that the size I wear? And how would I find out? What the heck does that 38 mean?" (I think it is chest size.) The coat showed up and the sleeves were not quite long enough. I only bought it for a Halloween costume, though, so I figured I would just resell it or donate it when I was done.
Remembering sizes is sometimes a wasted effort, because sometimes those sizes will fit differently depending on the manufacturer. Trying to remember a different size for every manufacturer seems like it is about ten times more information to memorize, so why bother? Just try shit on and see if it fits. Women's sizes have a bit of notoriety in this regard, and a woman can fit into any of three or four different dress sizes depending on the manufacturer.
Then there is the added difficult of weight gain changing the size we wear. I used to fit pretty well into a pair of pants with a 30 inch waist. Then it was 32. Then I just started buying 36 and wearing the pants with a belt. (Maybe gaining weight is why the 38R doesn't fit me so well?) We're buying new uniforms at work, and my manager wants to get my measurements for things like my collar size. I do not know my goddamn collar size, because I never buy shirts that have a collar size. Okay, actually, I did this once, back in 2006, and I can't remember which number I went with.
So, go forth and memorize these things about yourself. And if you're not sure you can memorize them, write them down and keep it somewhere you can remember it. Also include things you like but forget that you like, such as food, beer, and wine. Trust me on this. You'll thank me later.