Don't use a custom buffer size. Let OBS choose the correct size by itself.
Your post indicates you don't know what setting is appropriate, thus any value you set is worse than letting OBS choose the best size according to best practice coded into OBS.
By setting a larger buffer size you probably wanted to compensate for bad network connection. Unfortunately, you cannot just make network problems going away by doing this. Fix the network problems at the core is the way to go.