What is the most standard subnet length on internet

I would like to ask question more specifically.

I know most of tier1 providers accept their customer prefixes smaller than /24.
it means smaller than /24 routes always routable in that Tier1 provider.
What about between Tier1 and another Tier1 ?
Do they exchange their full customer's routes without any limitation of prefixes length ?
or do they refer to some DB information like Regional Internet Registry and
apply another filter on neighbor configuration with another Tier1 ?