Hello @AnjinMaru! Please build the baggage carousel outside your secure zone. Baggage carousels are supposed to be placed outside secure zones.
As for the check-in ratio, each check-in desk has a service cap of 75 passengers. So build ample check-in desks to make sure all passengers of a particular flight can check-in
This link lists the maximum capacity for nearly all types of planes that can be found in the game.
Here is a calculation by @twocflyer that is based on the above values.
As for the baggage ratios, I’m not aware of anything such. Maybe this topic might help you.