Hi, what size of optical audio is it in mm?

it's this type

http://en.wikipedia.org/wiki/TOSLINK

same size / connector as

my playstation2

my sat disc box

my DAB radio

my Sony surround receiver

which makes me suspect that if there are other sizes it's not

widely used in consumer electronics

tl;dr

the wiki link should provide with physical specs

Standard optical connections are all the same size. Mini optical connections exist, but you won't see them all that much and should be quite obvious when/if you do.