I am pretty sure there has to be some sort of idiom that means "fits perfectly," since trying on clothes to see if they fit is something that everyone does. Is there any such expression?
One such idiom is fits like a glove:
to be the perfect size and shape for someone:
I love these pants because they fit like a glove and they're so comfortable.
It can be used for any type of clothing (although you would only say it for gloves or mittens ironically).
Perhaps this one is almost too obvious, but:
The [article of clothing] is a perfect fit.
would be the most natural way of describing this.
You can Google expressions such as "sweater was a perfect fit" or "jeans were a perfect fit" and find this phrase used hundreds of times in online clothing reviews, such as:
I couldn't be more pleased. Not only was the service absolutely outstanding, the sweater was a perfect fit.
These jeans were a perfect fit and extremely comfortable but the flare leg is not for me.
This shirt was a perfect fit; it wasn't too big or too small. The shirt is also very comfortable and I can wear it with almost anything.