Hi DirtMinx
Clothes....It would depend on just what size we are talking about. But it is true that larger sizes are more limited than smaller sizes. There are lots of Dresses and skirts (beach type) that are a free size, Some tops may be hard to get in larges sizes. It will be hot when you are there so don't bother taking anything warm, you will live in lightweight summer clothes. So it would be probably wise to take some clothes with you but don't go overboard. It is really cheap to have washing done at one of the hundreds of street stalls 50 baht per kilo. Drop it off one day pick it up the next.
You wont have any dramas taking panadol etc with you as long as you are not taking the whole chemist shop worth. I always take Panadine with me in the large box 48 tablets. Never declare or anything its not a drama. You will be able to buy anything you need over there chemists are great.
A lot of hotels have power adaptors available but I always take my own. I have seen them in the stalls but I would get one at the airport on your way out just to be sure.
Not sure about dirt bikes but there are Quad bike (ATV) adventures over there that are lots of fun. Check out CathyP web
page it will answer many of your questions even the ones you have not thought of yet.
http://www.cathyandgarystravelpages.com/Enjoy your trip
Michelle