We line up our pork buyers BEFORE buying the weaners, so we know we won't be left with surplus. We normally get 3 weaners, keep 1 for our own use and sell the other 2 to friends. We sell either as a whole pig or in halves.
Weights will depend on what breed you have.
Don't think you can sell to an abattoir but you might be able to sell to the butcher if you don't have private buyers. Talking to your chosen butcher would be a helpful first step for you. We have a good relationship with ours and he collects our beasts (both pigs and sheep), takes them to the abattoir and then we go over to the shop once the carcasses come back and look at them, discuss what we could be doing better regarding feeding or whether we've got it right and how we want it cut up.