What settings should I use for buffer size in both applications?

Both Scratch Live and Ableton Live offer the ability to adjust audio latency to get trouble free audio playback. You may need to adjust one or both of these to get a good balance. Increasing the latency in either program will reduce the load on your computer's CPU and free up more of your computer's system resources.

Scratch Live
Try adjusting your USB buffer latency in the Scratch Live Setup screen. The lower the latency the more system resources will be used. Try adjusting this to a higher value to lighten your processor load.

If you are requiring a low latency in Scratch Live for DJ techniques such as scratching and you are experiencing issues, then you may want to keep Scratch Live latency low and increase the buffer in Ableton Live.

Ableton Live
Increasing the USB buffer in Ableton Live increases the time between changing a controller device and the application responding.