It's a rating. Like tires.
Go shopping for tires. Almost any tire these days is rated 112 mph. *You're allowed to drive 65 on those, it is saying don't exceed 112 mph*.
It's the same with subpanels: The "100A" is a maximum rating. Do not exceed 100A.
However, you certainly should exceed your feed-breaker size of 50A. Even if 50A panels existed, they would be very small in terms of numbers of spaces. The most important job of a subpanel is to not run out of spaces. So you should be buying a subpanel with way more spaces than you think you're going to need, to assure you never run out.
So size your panel for the spaces and make sure its busing is at least 50A (which won't be hard).
What about sizing the main breaker?
Yes, that means you may have a 150A breaker in the subpanel being fed by a 50A breaker. That is fine.
The breaker isn't there to be a breaker. It's only there to be a disconnect switch, to satisfy a Code requirement for a disconnect switch in an outbuilding. Feel free to price separate disconnect switches if you want to, but you'll find "just getting a panel with a main breaker" is the cheapest way to get a disconnect switch.
"But I'm clever. I want the breakers to be the same size so the breaker in the shed will trip first, and save me a long walk to the house". Sorry, breakers do not work that way. Even if you found a way to match breaker sizes, realistically the feeder breaker in the house will trip most of the time. Especially when it is raining!