What size pipes does this fit - or is it able to fit everything and you just modify the electronics for the size? Could it be used on corrugated pipe - with less accuracy perhaps?

First of all, we need to clarify a few things. Your original inquiry was for a flowmeter to mount over a flume. In that case, we'd be talking about an open-channel flowmeter. The flowmeter is literally suspended over the flume itself in order to measure the water level; no pipes are involved.

Now, if you want to mount the flowmeter onto a pipe instead of mounting it over a flume, then you're talking about one of the closed-pipe flowmeters. These require a few things: First, the pipe needs to run full in order for a closed-pipe flowmeter to work. An open-channel flowmeter can work with zero flow through the flume, but a closed-pipe flowmeter needs to have a full pipe.

Second, the closed-pipe flowmeter needs smooth pipe to work, not corrugated. The closed-pipe flowmeter works by firing off a Doppler ping and measuring how much and how quickly the signals come back; for pretty obvious reasons, that doesn't work with a corrugated pipe.

I hope this clarifies your questions a bit. If you have any others, please let me know and I'll be happy to help.