Keep in mind that all of the ...

Keep in mind that all of the responses talking about what the velocity should be, typical values, etc. have nothing to do with answering your basic question.

If you know flow then you have to have either the inside pipe diameter or the velocity. Don't cloud the issue about other things that have nothing to do with your question. Either somehow measure the diameter of the pipe is typically the only thing you can find out where you know the flow. Simply you could wrap a flexible tape measure around the pipe and calculate the diameter from the circumference.