there are two different types of birdhouses. Nesting boxes are for a pair who are sitting on eggs and can hold four or five babys and the parents and the hole should be small enough so larger birds can't enter and no perch near hole where bigger birds can perch and reach into the box.
The other type are roosting boxes for winter time or cold climates where the birds can stay at night or during bad weather. they are usually larger and often with two or more openings and holes sizes according to which birds are wanting to roost. often birds such as chiskadees and nuthatches and tits ect will share a roost box. if you get lots of birds in one box inside perches work and the younger birds are usually close to full size by the winter months.

It depends on how rough it is. You would only need the kerfs on the front wall on the inside of the box. Sometimes I just use a nail and make some deep horizontal scratches in there. Tree Swallows will definitely need them. They have a hard time getting out otherwise and sometimes die in the box never fledging. You can learn everything you will ever want to know about hosting cavity nesting birds on www.sialis.org This website is great and will save you a lot of heartache. There are predators, parasites, non native invasive birds etc that can kill your birds, eggs, and nestlings. You have all winter to research as nesting season is just about over for this year. Good Luck
Cathy

Here is a link that might be useful: sialis.org