The winches sold by Howes and Component shop at remarkably low prices have 1" drums, so a 4 turn one gives about 15" of travel. This is governed by the transmitter, different makes/models/batches will vary a bit. Translating line travel to sail movement is largely a matter of tweaking the geometry of the relative positions of the boom fitting and the deck fitting. 6Kg is plenty for a Victoria (30" long, about 200sqin of sail, so the same should be OK on a barge, but for the extra quid or so, a bit of extra pull will do no harm.
When setting up, just make sure that the line has a nice straight run from its shroud outlet to wherever it vanishes into the hull structure. I use some nice braided line about 30Lb, but I've had it for years, the label has fallen off, and I can't remember its full name.