If we're talking about conventional nylon, a 3/4 oz spinny is ideal for use up to 18 knots apparent.
Realize that just as genoas come in different sizes (150% = "Number 1"; 100% = "Number 3"; et cetera) so do assymetric spinnakers. For cruisers, a G3 (3# Gennaker) is a great chute, highly manageable by a shorthanded crew and far more versatize in reaching conditions, especially in a moderate breeze.
In other words, don't just look at the luff. The foot can be an equally important measurement.
One final thought: a lot of people buy used chutes because they rarely use them. And then end up rarely using them because they don't have an ideal sail. Getting the right sail fit to both your boat and your expected uses, you might find that you use a well-made chute a lot, and the savings of fuel
in light airs will soon pay for the initial expense for someone making long passages. I constantly see people motoring downwind in light air because they haven't got the sail they need to do it right.