I have six 500GB SSD drives in Raid-5 using LSI 9271-8i Raid Card. This server is running Window 2012 Hyper-v, it will host about 20 VMs. The default stripe size is 256KB, should I use larger size like 1MB?
As usual, to answer this question you should ask yourself what stripe size is and what does it impact to:
The smaller it is the closer data is interleaved and more smoothly (equally) workload is being spread, but it also means more devices and (more often) would be involved when doing even small I/O — read "busy".
The higher it is, the less benefits of load distribution would have theirs chance to happen when dealing with single file (it could sit entirely on one disk only). But it would keep other disks free, so they could process requests to other files at the same time.
My usual choice is large chunks (stripes) and I think for "20 VMs" scenario it should be exactly the same.