The ball can go either between or slightly behind (more comfortable) the teeth, and I get the feeling most people will be recommending the Aslan gag if it comes back in stock. That particular one is, I believe, 1.5 inches in diameter and remarkably squishy while still being really durable.
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If you'd rather not wait for it to restock there's also the Fetish Fantasy one that's the same diameter.
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Or this one, which is 1.5 inches just like the other two.
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Personally, I have and love the Aslan one myself and I'd say the quality of that gag not only more than makes up for the price tag, but is also worth the wait. You can always submit a support ticket here if you want to ask when it'll be in stock again.

As for materials, silicone is the best but I've had luck with the Fresh Ball Gag not having a nasty rubber taste even though the ball is made from rubber. It's a matter of personal preference, but silicone is much more hygienic and can be used even if you have a latex allergy.

Tips on measuring, I think I just opened my mouth as wide as I thought might be comfortable and used a measuring tape myself, but keep in mind that the longer the gag is in, the more uncomfortable it can (and likely will) get. So a 1.75 inch ball for just half an hour could be about as comfortable for that shorter time as say, a 1.5 inch ball would be for an hour or more.