I had mice in my garage last fall. I have also had pet mice. They can fit in any space they can fit their head through. They are also very good climbers. I sat in the garage and watched them climb up electrical cords and do a tight wire act on the top of the peg board. They are pretty talented and amazing creatures. They will only stay if you feed them. I put my bird feed and saved seeds in sealed containers, and they moved on. They hung out at the neighbors for a while where they would steal food from the dog, and burry it in the back yard. They like grains and seeds and sugar. Is the yard overgrown with some plants gone to seed?
My parents have a farm, and as long as they had cats, they did not have mice problems. (if you like cats)