If you're worried about potential farrowing problems because of the difference in size between the sire and dam, don't. The sow determines the size of the piglets at birth, not the boar. However, it's likely that the piglets may eventually end up bigger than pure-bred piglets from that sow would.
Fortunately, it's not like with sheep, where big heads and shoulders inherited from large sire rams cause lambing difficulties.
However, I would question the wisdom of the seller putting a LW to KKs - not least because a mature LW would be a fair old weight for a KK to support on her back (unless this one was particularly small or very young).
From a meat point of view, the LW x KK should be a good combination - but I wouldn't recommend repeating the same experiment with a large boar and smaller sows.