@martin-smith

The tempting response to your question is: How long is a piece of string?

IOW, there can really be no recommended buffer size since the actual setting will depend on so many factors inherent to your system, at least some of which may be unique to you. Your computer hardware, software, audio interface etc., etc. will all influence what the audio buffer needs to be set to on your computer.

As a general rule, if you are overdubbing the buffer size should be as small as your system can handle without problems being manifested during playback (crackles, stutters, dropped samples etc.). For mixdown, the buffer can be increased to allow your system time to process the throughput of all the data, without creating a "delay" between pressing "Play" and hearing the audio.

HTH

Jeff