On Sleeper, you can always draft or trade over the roster limit!
If you have three open spots on your bench heading into your rookie draft, but you have five picks, don't worry. You can still draft all five players. You'll just need to cut your roster down to the league limit before it becomes eligible again.
Sleeper leagues do not require rosters to be under the roster limit before a trade or draft is completed.
Team owners who are over the limit will see something like this at the top of their team page:
When a team is over the roster limit they CAN:
Draft
Drop players
Trade
Accumulate points
Move eligible players to IR and Taxi Squad
When a team is over the roster limit they CANNOT:
Add Free Agents
Edit their starting lineup
A team owner does not have to be at or under the roster limit in order to gain points toward a matchup. They will not be able to adjust the lineup, so it would be locked.
Why do we allow users to go over the limit?
We call this behavior "lazy enforcement." After introducing this feature, we saw trade activity triple.
It's something not many fantasy platforms think about, but it makes a huge difference in league activity thanks to the added flexibility it provides.