My son owns a Chihuahua. He was given the Chihuahua by his nana. At the time I owned a Bernese Mountain dog. They were great friends and the Chihuahua used to sleep wrapped in the Berners tail. Sadly I lost that Berner 4 years ago.
I own another Berner now and she is great with the Chihuahua. Very gentle. When she plays she lays down so she's on his level. In fact we just had my MIL's dogs stay, a Basset Hound and a Goldie and they got annoyed with the Berner as she doesn't know how to play with larger breeds.
Good luck with your decision.