What size half chain link do you need if i get a 25t sprocket?
The chain length needed is determined by chainring size,
sprocket size and the design of your frame. There's no way of
saying that you'll need from the sprocket alone. On top of that
there isn't much to choose from when it comes to half links. They
are all of the same length and in width there are two options 1/8"
or 3/32". You need one the same width of your chain.