Some pacifier have different sizes while some don't. Is it necessary to change pacifier accordingly?

Yes pls choose the one for the correct age range as it caters to the shape and the growth of the oral cavity and possible teeth

it depends. as baby mouth gets bigger, they may want a bigger teat to they can suckle better'

Yesthose that have different sizes is suited for babies of the age. It's specifically made.

I've not seen pacifiers without sizes. What brand is that?