I can't help with the Java implementation but I've done it a couple of different ways and you don't really set the buffer size - you set the number of bytes to read from the buffer.
If I need to verify that the equipment is responding with the correct characters, I will first set the term char enable off. then, after doing the VISA Write, I will use the function to determine the number of bytes in the buffer (VI_ATTR_ASRL_AVAIL_NUM). I will pass this number to the VISA Read. I will scan the string returned for whatever single or multiple character I want. This is in a loop and if the expected character(s) are there, the loop stops. If they aren't, then the loop continues. I would place a timeout function (separate from the VISA built-in timeout) and return an error if the character(s) are never seen. The character(s) are a string that I pass to this general read function.
Another technique to use is to simply not care if the instrument is returning all of the correct acknowledgements and simply read whatever is returned. In this case, I have one loop with the VI_ATTR_ASRL_AVAIL_NUM function and exit the loop when the number is greater than 0 (there is also a timeout function here as well if the instrument never responds with anything). Then, another loop is called and this also has VI_ATTR_ASRL_AVAIL_NUM to pass the number of bytes to the VISA Read. This loop exits when the number of bytes is 0.
Which technique you use is up to you. There is more error checking in the first technique. The second is more like how Hyperterminal works. You just get whatever the instrument sends.
