Kiteboarding on Nantucket is like a lot of other places, in that the season you visit will make a great difference in the type of conditions you are likely to run into. Also it is typical East Coast, so there is a chance that you could get skunked on a one week trip but probably not for a two week, depending on the season.
Summer on Nantucket is all about the sea breeze. We get a lot of good South and South West days and they are usually the gems of the summer. There can also be a killer North East in the summer but that is definitely the exception, the rule is the south.
The most typical kite size used in the summer is somewhere in the 12-16m range depending on your size. I am 200lb writing this and I always sail my 12 the most in the summer. Like most places, you want to have your big kite with you. We definitely have windy days and ride 8s and 10s in the summer but if you were coming from off island and brought your 16 and 12m kites for a trip in July or August you would probably be ok.
Like most places most good riders are riding typical twin tips in the 125-140cm range. The summer conditions on the inside are mostly flat water or small chop. There is good ocean riding but it is usually a bit lighter on the ocean side than on the inside and the ocean really doesn't get the good thermal effect like the harbor does. But if you have a surf board you can get on it if you make an effort.
The water temp is comfortable (I think like 65-70°?) but remember this is New England, most riders will be wearing 2-3 mil shorties but later in the day some full suits might be seen. On a killer sunny, warm day guys will ride with just a rash guard but usually not for the whole day. The ocean is a little bit cooler.
If you can get your hands on a boat that is very helpful. This is more true in the summer than any other time.
The spring season is marked from when you can comfortably ride without booties (before this is the pre-season). This is usually sometime in late March or more likely mid April. The April - June season is usually really windy on Nantucket. You can often get a lot of 9 and 12 days (a full sized guy - so 8-10 days for a lighter guy or girl). Perhaps because it is usually windy and the water is still pretty cold there is less interest in riding big kites but of course it does happen.
The water is still pretty cold so bring a 5-4 or 4mil suit or a nice dry suit. Often the water will be a lot colder than the wind, especially on a really nice day. Ocean directions like SE, W and NW are more likely to blow during the spring and fall so this is the time to look seriously at the south shore.
For most kiteboarders on Nantucket the season from late September to November is comparable with the best riding anywhere. The wind is usually really good, a lot of solid 12-9m days will be had by full sized guys. The weather is still awesome and the water cools off pretty slowly so it is very comfortable late in the season.
Bring your full quiver but don't forget your small kites. Every fall I get 7 or 8 days on my 9m kite and a few days on the 7m as well. There are a bunch of days where most kiters are blown out. The water is pretty warm so most riders will be in 3 mil full suits and then transitioning into 4 mil or dry suits later on in Nov. and Dec.
Hard core locals try to get a few days in every month even the coldest winter months can produce a sweet 50° and sunny day for quality riding. It is a full rubber situation, 5 mil winter suit or dry suit, hood, boots and mittens. You have to be pretty into it but if you have a good tolerance for cold water there are several pretty safe spots where you can get a good session in the winter. Usually you would be on your 9-12m but there are so many unsailable, cold windy days.