I just worked up this small (advancing to medium) bat companion as an option for a bat that isn't going to go to Large size.. It's based on the Hawk/Eagle/Owl bird, but advancing in size instead of only the +2 Str, +2 Con option.
Bat
Starting Statistics: Size Small; Speed 20 ft., fly 40 ft.
(average); AC +0 natural armor; Attack bite (1d4);
Ability Scores Str 10, Dex 15, Con 12, Int 2, Wis 14,
Cha 6; Special Qualities blindsense 40 ft.
4th-Level Advancement: Size Medium; Attack bite (1d6);
Ability Scores Str +4, Dex –2, Con +2.
Bird (Eagle/Hawk/Owl)
Starting Statistics: Size Small; Speed 10 ft., fly 80 ft.
(average); AC +1 natural armor; Attack bite (1d4), 2 talons
(1d4); Ability Scores Str 10, Dex 15, Con 12, Int 2, Wis 14,
Cha 6; Special Qualities low-light vision.
4th-Level Advancement: Ability Scores Str +2, Con +2.