This time of year it's usually non-stop wind an all of the west coast, with the occasional trough, where the wind will just almost stop for a couple of days then return with a vengeance. When the wind is on, you'll fly a 7m 80% of the time and sometimes you'll wish you had a 5m.
Agree with Cauncy and Dave W, Dongara is the best of all worlds but there are many places to explore -- Lancelin and Cervantes also great spots.
One thing to keep in mind if you plan on being anywhere north of Shark Bay is that late Jan is the peak cyclone season, so you need to have a contingency plan.
Exmouth and Coral Bay are spectacular but don't try and drive there from Perth if you only have a week. Not by any means certain there will be a cyclone during this time, but your odds are probably one in four or thereabouts. When a cyclone forms, all the normal weather conditions will go out the window while it's around, and the effects can hang around for 10 days or more and you could end up not kiting. On the other hand, if you get it good, it's one of the best places in Oz to kite.
www.bom.gov.au/cyclone/Keep an eye on this page:
www.bom.gov.au/wa/forecasts/nwcyclone.shtmlShark Bay is awesome for flat water, south of Denham.
Around Geraldton it's Coronation, South Coronation and the best place is Oakajee, but it's accessible by 4WD and on low tyres only. Forget Gnaraloo, you'd need at least 10 days unless you want to just drive straight there from Perth and spend the entire trip there.
If you only have a week, you should probably fly to Perth and spend the week in Lancelin, Cervantes and Dongara and save the other spots for another trip when you have more time.