Can I change the input and output flange from 4'' to 6''? What is the size of the pipe for the drain?

No you cannot change the input and output on the flanges but you could use duct increasers to 6”. The drain is standard pressure fitted (push on) connector. It's held inside the HRV unit with a built in hole in the drain pan. The dimensions are 2 inches long with about a ½ inch diameter... If you have any further questions please feel free to give us a call! Thanks.