Please see this other post for an answer to the question in the subject.
Within the body of your question you indicate that you are running into a 255 character limit. When I was talking to one of the PI Web API developers about how to query multiple PI Points or Attribute values at once, the query / URL length was my concern. I was told the limit is roughly about 80,000 characters. So I am kind of surprised about the issue you are experiencing also because I am pretty sure that I have executed queries > 255 characters length.
What technology / object are you using to access PI Web API. Can you see if there's maybe a client side limit?
I was trying it from my python script. Let me try that in a browser first. Thanks