Full grown Bats only need 1/4 inch x 1 1/2 inch slot to get into, Bats flatten themselves out and squeeze through openings, I've seen smaller Bats squeeze through smaller slots. Bats don't chew or claw their way in Bats find existing openings. That being said Bats can find numerous places around a home they can enter from, missing caulk/mortar, cracks in wood, valley ends, short or unsealed overhangs, loose siding and soffits, wood thats missing or rotted out... Most places you can't see from the ground. Thats why its best to have a Bat Specialist come out and thoroughly inspect your home and find All the openings Bats can enter from.
Once Bats have found a way into your home the Bats are gonna live there for many years to come if you don't do anything to correct your Bat problem. Once you have Bats living in your Attic they can now end up in your living space. When the Bats are in your attic they can feel different breezes so they start to explore trying to find other openings thinking its another way out and every now and then the Bats find their way into your living space. Usually through cold air returns, unsealed plumbing under sinks, exhaust fans in bathrooms, down the sides of pipes and wires into the basement...
Alot of people with fireplaces believe Bats come down the chimney, which does happen but not very often. Bats can't Fly straight up so where ever Bats are getting in at the Bats have to be able to crawl back out of and Bats have a hard time climbing up the chimney Flue. That being said I have seen Bats going in and out of chimneys before, usually through a crack near the top of the flue.
Hope this Helps,
Bill
NEX-GEN BAT REMOVAL