How should I adjust the buffer size?

By lowering the buffer size setting, the latency (the period from when the DDJ-400 is operated until the sound is output) can be shortened. If there are interruptions in the sound, increase the buffer size, but note that this makes the latency longer. Set the buffer size to the minimum value at which no interruptions in the sound occur.

The buffer size can be adjusted in [Preferences] of rekordbox.