Where I can find Mongoose OTU information on nobles..

[Nerhesi]

Specifically, what kind of land/space grants they receive or generally have based on their rank. E.g - what is an average knight's holdings, vs a baron's and so on

 

[rust]

The Dilettante supplement has a little information about the
Third Imperium's nobility, but the best source is probably
GURPS Traveller Nobles:

http://www.sjgames.com/gurps/traveller/nobles/