Read and write cache is not enabled. Why would I use it. Recommended memory size?

You wouldn't unless you are experiencing problems or have old, slow drives. It forces Sonar to use the Windows disk cache for read/write operations but generally that isn't what you want.
 
The buffer sizes are relevant and can affect dropouts etc. I use trial and error to find a size that works for me with my latency settings. 512 is a good starting point and if you aren't getting lots of dropouts don't worry about it. If you are, try making them smaller or larger to see if that helps.
 
They are the buffers that Sonar uses for read and write operations. Too large they take too long to fill and starve Sonar of data, too small and they can't keep up. Either can cause problems.