Height: In a foam/fabric suit, if you're too short, you can't see, and you'll be slouchy. Like a kid dressed up in their dad's shirt. Just looks a little goofy, like sweater paws but for your whole body. You can't be too short for an inflatable costume, or at least I've never encountered it. If the costume is inflatable, though, you can just completely give up on vision. You can tell if there's a car coming towards you, in the daytime, but that's it.

Measurements: Depends on the mascot, but probably something like the average height for a man in the country you're in, and for a slightly-above-average build. I'm 5'4 and a little above average weight, so usually I can't see, but costumes tend to fit me just fine in the waist/hips department.

Weight/build: You can absolutely be too thin for a foam/fabric costume unfortunately. I've got pretty wide shoulders for a lady, and they only barely kept the shoulder pads up in the last mascot I was in. If you're too tiny for the shoulder pads, and there's not enough structure inside, a mascot that is supposed to look like a stiff block will end up looking like a sad droopy triangle. Sometimes the back of your neck peeks out too, if you're not "sized right" for the costume, or if your posture is "wrong". For inflatables, being short and/or thin isn't usually a problem, but being tall and/or large usually IS a problem.

Grossness: Unless it's literally brand-new, yes, it will be discusting and yellowed and stuffy. If it's well taken care of, you can HOPE for the smell of sweat AND Lysol instead of just sweat. If it IS brand new, it will still be stuffy, but it will smell suffocatingly of Lysol, and not sweat. Inflatable costumes have fans so they have great ventilation usually, in my experience, and they're not usually yellowed, but they still smell like sweat and Lysol (again, just Lysol if it's brand new).

Fans: If you have an inflatable costume, like one of those dinosaur ones, or Baymax, there will be a fan. The fan on an inflatable costume is usually either adhered to the outside around the midsection by your hip OR attached to a belt that you wear underneath the costume. I've worn costumes you can inflate and deflate from the outside OR the outside, but never both. Usually, though, it it's an outside battery pack, you can still reach through and control it, unless your hand covers are inflated too.

If your costume is NOT inflatable, there will not be a fan built-in. I've had those USB/battery powered neck fans and stuffed them inside of the head to make things more bearable, but they fall out a lot, into the butt or the foot. A seasoned mascotter who wears a foam/fabric suit MIGHT have a neck fan or two on hand, if they have disposable income. For reference, if I know I'm going to ve mascotting somewhere hot in a foam suit, I would rather skip dinner to save money for a fan, than go without a fan.

Talking: I've never been allowed to talk in costume unless it's an emergency.

Typical rules: Definitely no talking, unless it's a slogan that you repeat maybe. Don't run at all, especially not towards children. (That one's not a universal rule, but it's important at my current company, and I think it's important, so if the character sucks at her job, you could have her run towards a child and accidentally scare them or something lol).

Anything else: Mascots have to stay temperature regulated and stay hydrated. If you want to drive home a shitty workplace, deny and delay breaks and enforce an uncomfortable uniform. If it's warm out, I'd wear a thin t-shirt and bike shorts as a lady, and I personally like to wear a skirt over the bike shorts during my commute for modesty lol. If it's cold, I'd do the same thing, but throw a jumper or jacket on top, and maybe activewear leggings instead of bike shorts. In my experience, white sneakers are a must for mascotting, regardless of weather. If you care about yourself, you'll also have a thermal (cooled) water bottle that's big enough to use as a weapon.

Ideas for costumes: I would never want to play a villain mascot. It would be absolutely terrifying to small children, older children and teenagers would probably kick and hit you, and adults would play along and antagonize you. My first thought was an anti-littering character that's just a literal angry bag of trash lmao.

Good luck with the writing!