Are you hearing pops and clicks? Are you receiving CPU overload error messages? Hearing latency? Changing the buffer size could fix the issues above as this is the amount of time allotted for processing.
Here are a few scenarios of when you would want to adjust the buffer size:
When you are tracking, you should lower the buffer size to at least 256 samples to reduce the amount of latency you hear when monitoring.
The disadvantage of a lower buffer size is that this puts more pressure on your CPU and makes the processors work harder. A tip to fix this is to use fewer plug-ins during the tracking process.
When you are mixing, you won’t need to worry about latency because you won’t need to monitor any live signal.
You can raise the buffer size to the highest setting as this will enable you to utilize more plugins/alleviate pressure on your processor before encountering any CPU messages.
If you have your buffer size all the way up and hear a small delay when starting playback in your DAW, this is normal and not an issue with your DAW or interface.