Hi,
what should be the minimum specifications for a servo used on 26cc model for full 3d capability.

Full 3D on a 26cc petrol??  . Must be interesting.

Question.... 26cc petrol engine is heavy for the size of the aircraft it is usually mounted on ?  Is that why this is interesting ? 

Yes. thatz right.

Regarding the servos,  I would personally use Hitec 5645MG for all the control surfaces.  That should be sufficient! You might even consider using a Hitec 7955TG for the rudder.  It would definitely be an overkill servo (and also expensive), but you can again reuse it in any bigger model too as a rudder servo!! The best way to get a 26cc plane to hover is to get a .90 size airplane and dump the 26cc petrol engine on it!! That should make it do the job!!

- Chan

Although not on aircraft, I use HS-5645MG on both my Baja 26cc and 1/8 Nitro. Both cars, specifically the Baja tipping the scale at 10kgs need lots of torque to brake.

Not sure about other brand equals, but HS-5645MG is a great value for money digital you can get in Hitec. Gives decent torque/speed for the money you pay.