I use a bar end mirror on my road bike. It isn't small, but gives a good view of what is behind. As the first review on the link says, do have to remove the mirror from the mount when you first mount it, and it is a tight fit, but once mounted, works very well and doesn't seem that vibration really bothers it. Inexpensive to boot: https://www.amazon.com/Sunlite-Ce-2-.../dp/B00B13686K