Before you go dosing them (perhaps without need) get yourself one of these worm count kits
https://www.suppliesforsmallholders.co.uk/worm-count-kit-p-2242.html All wormers are much the same - it depends which worms you have and whether you want to dose them orally or by injection. 6 monthly is the interval for worming - usually.
How big they get is up to you and how quickly they'll grow depends on the breed, the area they have to roam in and how much you are feeding them. But it's worth checking with the abbatoir that you intend using as some have cut off weights above which, they can't take.
To weigh your pigs - measure around their belly, just behind the front legs (in meters) multiply that number by itself then multiply by the length (from the base of the tail to the base of the ears) in meters then multiply by 69.3 to get their weight in Kg.
Pork pigs are generally around 85Kg liveweight and bacon ones over the 100 - but check with the abbatoir that they'll be able to take them if they are bigger.
HTH
Karen x