See ThreePhaseEel's answer above.
TPE is a panel master. My info is more general to panels at large, leaving it up for reference.
You're powering a shed. It's an outbuilding. That means it needs a main break-- I mean a main shutoff switch of its own. Generally the cheapest way to provide a main shutoff switch is to buy a panel with a main breaker in it. Also the most compact (as opposed to a separate shutoff switch).
That's not happening with this main-lug panel. You could backfeed a plain breaker, but hold on - Code requires bolting the breaker down so it can't tip out like a normal breaker.
Normally, when a sub needs a main shutoff switch, and you use a main breaker for that, nobody cares the breaker size. However, I note that some of ThreePhaseEel's scenarios in the other question call for tapping a much larger feeder, and that definitely requires an actual, workin-for-a-living main breaker.
With any panel, prepare for the tiedown kit to glom 2 additional spaces, as many panels have unusable space abeam of the backfed main breakers.