The limitation per case is 115cm L+W+D, and are here :
http://www.thaiair.com/Travel_Destin...%20to%20check?
Depending on how you look (or hold) at your suitcase, the length can also be the width or height !! Kep point is the sum of the dimensions <= 115cm, with L, , H and W being specified as 56 + 46 + 25 respectively. But I've never seen this become an issue unless the bag 'looks' too big.
Max weight for 1 bag is 7kg, but judging by the struggle to lift them, many weight much more!
Text here:
How many pieces of carry-on luggage am I allowed?
Each passengers may hand-carry one piece luggage the sum of whose dimensions (length+ height + width) does not exceed 115 cm. ( 56 + 46 + 25 cm) (44.85 inches, i.e. 21.5 x 18 x 9.75 inches) and which does not weigh more than seven kg. (15.4 lbs). Pieces exceeding these dimensions must be checked in at the Check-in counter prior to departure. A woman is allowed to carry a purse as well.
Biz and First pax are usually allowed 2 pieces that meet the above requirements, but I can't find where this is actually stated!.