Can you tell me what the connection size for an energy meter is?

It varies by each energy meter.  You buy the meter based on DN size and match up to the tails (pipe union connections).  The connections on the body of the meter are always ‘bigger’ than the final connection to the pipe.  When you use the pipe union connections kit – e.g. a DN15 body has a 3/4″ body connection – needs a 3/4″F x 1/2″M connection (02708098) – therefore if you want a 3/4″ pipe connection, you need a 1″ body DN20.