>What scale unit are the height and width fields of the frx table? Pixels?

Yes

>What scale unit are the Paperlength property in the Expr field of the first record of the frx in? Does Papaperlength include unprintable area?

I think it depends on your printer driver. For example, I don't have a Paperlength property in my FRX, I have Papersize=1 and I'm using the HP Laserjet 4 driver.

>How can we convert between pixels and inches?

I'm not sure there is a straight conversion.