I have done a write up about the 7" tablets-- I have had both a cheaper Chinese unit and a Nexus 7 -- read
http://www.myswag.org/index.php?topic=25989.0 --
Do yourself a favour don't do the cheaper Chinese unit -- go straight for a either a "ASUS Nexus 7" or a 7 inch "Samsung Tab 2" The pricing of those units are about the $250 mark and dropping rather quickly.
As for the Nexus 7 unit that is the unit that I have -- it's very quick to use, capacitive touch screen is excellent, easy to see in bright day light, where I have it mounted on my dash -- I have documented my mounting method in that writeup.
About size your tablet to get, in my opinion 7" is the best as any larger your going have issue either obscuring your view or blocking your cool air from your air con ducts etc.
I have not using been Oziexplorer much since I have discovered Orux Maps, so you might be able to save some moneys going straight to Orux Maps, for your raster off road navigation softwares.
Something I have noticed about the 7 inch "Samsung Tab 2" -- as it's Samsung Android devices it comes with a free version of Navigon
http://www.navigon.com/portal/au/produkte/navigationssysteme/index.html that's a voice guided GPS navigation app, that could save yourself from buying a few $$$ -- but on saying that I use Sygic on my Nexus 7 -- and Sygic is a better product than Navigon. (I have Navigon on my Samsung phone and Sygic is a better product but it costs $33).
