Hi

I traditional have all ways used via with a size 0.6mm with 0.3mm for all my pads and connections.

When it comes to connected a copper pour to either VCC layer or GND layer, is it better to use less via but slightly large in size or to use much more via but smaller in size?

Does it make a difference?

My PCB requires various stitching from a plane layers(VCC or GND) to TOP or Bottom layer?

 

it depends on what you are trying to achieve...
if you are considering current only, using few larger vias or bunch of smaller one can be the same
if you are using vias to form a shield around sensitive signals bunch of smaller ones is better
if you are looking at the PCB manufacturing costs, few larger ones is better.
there are various via calculators, check them out