Another good idea is to go somewhere you can try shirts on (if you don't have one) and find one you like (seams at shoulders, wrists covered but sleeves not ruffled) and look at the size. If worn with a tie, the neck should button and not be tight, but you should not be able to fit more than one finger in between the shirt and your neck, otherwise a tie will cause the neck to crumple when tightened.
Memorize or write down the size of the shirt, given in a neck measurement (inches, like 15 1/2) and a sleeve length (inches, and often a split value like 32/33). This will help if you enter a department store where the shirts are bagged and not easy to try on. Look for your size (neck + sleeves) and hope for the best. These numbers are good to know, as neck sizes may be sold with wide sleeve ranges (30/31, 32/33, and 34/36), and those buckets make a huge difference.
Lastly, find a particular brand that seems to fit well, if you can. I shop a lot at thrift stores and am of a narrower frame and really have a hard time finding 15-15 1/2 necked shirts with the right sleeves that aren't very "blousy" (where once tucked in, there is a huge "balloon" of shirt sticking out in the back). Pay attention to labels like "classic fit," "modern fit," or "athletic fit." Classic and athletic tend to be slimmer/tapered, and modern tends to be more of a static width, extending the width at the armpits down to the bottom hem.
I still have not figured out how to find work shirts that won't ballon when tucked in. I may be smaller than most people who give their clothes to the thrift store, or it may be there's something about this I don't understand.
I am beginning to suspect that it is surprisingly common for intelligent, competent adults to somehow make it through the world for a few decades while missing some ordinary skill, like mailing a physical letter, folding a fitted sheet, depositing a check, or reading a bus schedule. Since these tasks are often presented atomically - or, worse, embedded implicitly into other instructions - and it is often possible to get around the need for them, this ignorance is not self-correcting. One can Google "how to deposit a check" and similar phrases, but the sorts of instructions that crop up are often misleading, rely on entangled and potentially similarly-deficient knowledge to be understandable, or are not so much instructions as they are tips and tricks and warnings for people who already know the basic procedure. Asking other people is more effective because they can respond to requests for clarification (and physically pointing at stuff is useful too), but embarrassing, since lacking these skills as an adult is stigmatized. (They are rarely even considered skills by people who have had them for a while.)
This seems like a bad situation. And - if I am correct and gaps like these are common - then it is something of a collective action problem to handle gap-filling without undue social drama. Supposedly, we're good at collective action problems, us rationalists, right? So I propose a thread for the purpose here, with the stipulation that all replies to gap announcements are to be constructive attempts at conveying the relevant procedural knowledge. No asking "how did you manage to be X years old without knowing that?" - if the gap-haver wishes to volunteer the information, that is fine, but asking is to be considered poor form.
(And yes, I have one. It's this: how in the world do people go about the supposedly atomic action of investing in the stock market? Here I am, sitting at my computer, and suppose I want a share of Apple - there isn't a button that says "Buy Our Stock" on their website. There goes my one idea. Where do I go and what do I do there?)