As a general rule of thumb I'd say use 20 lb for 7 inch baits, 25 lb for 9 inch baits, and 30 lb for 12 inch baits.

You could probably throw the 7-9 inch wood baits on 14 lb test. But there would be a high chance of failure if your line had the slightest nick in it. And that's never a chance I want to take. Even without scraping it against cover, the first 2-4 feet of your line gets stressed just from casting these heavy baits.

Overall I don't think line visibility is a huge factor for swimbaits. But if you are fishing super clear water I'd use a florocarbon leader before downsizing my line.