Is it the PPC range of cartridges that uses the same base dia as the 7.62x39 russian??

yes, as well as the 220 russian.

I understand that there are a lot of wildcats made out of this case, what do you consider the optimum bore size for this case?? ( ie, 6mm, 25 cal , ect,)

Depends on your task.
22 PPC's are very good varmint cartridges out to 300 - 350 meters.
6mm PPC's dominate benchrest competition at 100 and 200 yards.
The 6.5mm PPC's (such as the 6.5mm Grendel) do a good job of bucking the wind for target work out to 400 - 450 meters.
I have a sporter weight Sako in 6PPC that is my walking around varmint gun. It is awesome out to 250 meters. After that the 70 grain bullets get blown around some.
Not many super accurate 25 caliber bullets.
Prepared brass is spendy. Making brass out of 220 Russian cases isn't hard. Making brass out of 7.62 x 39 brass is about as much fun as poking yourself in the eye with a sharp stick.
JCN

I was afraid you'd say that, I have a shitload of 7.62x39 brass left over from an attempt to make an SKS shoot before they all got confisated.(that didn't work either, trying to make an SKS shoot)

CZ makes a dandy little 7.62 x 39 carbine. It is too much fun, and would work great on Roo and short range donkeys, dogs, and rabbits. Cheap to feed also.
JCN

I haven't seen Charles Daily available in Australia but the Zavasta is what I am looking at.They come in 7.62x39.

Get it. You won't be sorry. Load all your brass up, shoot it all, and do it again. Not much recoil, not much money, lots of fun. Down the road you can always rebarrel it for something a little more glamorous.
I love my 6 PPC walkabout rifle, but whenever I shoot it I end up on all fours in the weeds looking for my $1.06 USD per piece brass.
A 2.5 power scope wood work really well with the 7.62x39, nothing more than 4 power.
Go forth and slaughter the varmints for God, the Queen, and Miss Universe (not necessarily in that order).
JCN