The buffer size is kind of like a speed control - it's the number of samples held over before feeding audio in and out of the soundcard driver. As it's the number of samples, how often transfers occur will depend on both the size of buffer and the sample-rate of the audio.
The smallest buffer (64) may work, but there is more chance the process will get interupted by other event in the pc (resulting in pops and clicks) and it increases the number of interupts from the card itself and shows up as higher CPU usage.
A larger buffer is safer allround, but the delay (latency) between events and the audio actually being heard is longer which matters if you're playing softsynths or monitoring the recording from your softwares input monitoring.
If you don't need super low-latency, 512 is a reasonable safe buffer - which is why they make it the default. 256 is a good compromise between latency and CPU usage in my experience with soft synths.
The lowest value you can use without clicks would be the thing if you rely on input monitoring from your software though, as this requires a 2-way trip for the audio and therefore two trips through a buffer.
