Wind below 10 knots is a non-factor.
Beyond that, there are really no good rules of thumb to tell distinguish between go/no-go -- just the extremes are obvious. Too much is boat specific and one guy's 4 feet at 8 seconds is another guy's 5 feet at 5 seconds. The best thing to do is go one when the forecast is less than 4 feet and the period greater than 8 seconds and see how you and your boat handle it. Make note of the conditions, and see practice estimating size and period. When you get back, check what the actual size and period at your location were (you might be surprised). Once you establish a base line, be a little adventurous. On a prior boat, I went out of the harbor into breaking 10' waves. (The thing about the ratio of height to period is that it is a good measure of the steepness of the waves -- it really doesn't get any steeper than breaking, and absent significant wind, you are not going to see breaking swell in the open ocean unless the period is way shorter than wave height.) Anyway, that is the worst I have ever been in intentionally, and I will do whatever I can to avoid an 8 foot swell with even an 8 second period, knowing that the forecast could be easily be off enough that actual conditions are 10' at 6 seconds.
