Reliable is the rubber valve. Ceramic not so.

Pipe size.

Depend where the incoming. If up, stop cock will be on top, if bottom stop cock will be on the bottom.

It is the pipe size and the tap, stop cok etc joint size. If you buy the plumping parts, you need to tell them the pipe joint size.

For rule of thumb, main inlet till tank is 3/4". While those toilet and kitchen is 1/2".

So does it mean that this stop cock in my bathroom (first posting) is 3/4" since it controls the valve for the water tank?

I suppose I can remove the stop cock knob cover cap (the one with blue dot in the middle) and replace with a nicer looking cover cap?

The water coming to your toilet from the water tank is call outlet. Which mostly is using 1/2" pipe. While from the meter outside to the water tank is 3/4" pipe.

The stop cock in your bathroom is to stop the water coming to the toilet tap. Since the pipe is coming from the tank and to your toilet, it is an outlet pipe. And size 1/2".

You cannot simply replace the knob. Each brand have it own design. Each brand can replace back it own brand parts. Unless those common parts. Ex. rubber washer.

Maybe those expensive type like grohe, kohler etc have an option to change the cover and knob design. But still have to using back it own brand.

So you have to hack it out and replace the whole stop cock.

I see... So for Malaysia pipe standard is for 1/2" inside e house.

Is it that for 1/2" inch, the female thread diameter in all wadever water tap or valve is about 21mm? The reason why I am asking is that I would like to purchase tap fittings from overseas and their standard is G 1/2".

Pipe plumbing is normally in inch. Female or male thread, just mention what inch you want. There is no mix of inch and mm. G or NPT is either straight thread or taper thread.