I've been to Southeast Asia on vacation (including Thailand) 3 times in the last 3 years. I've found that it's fine for women to wear long pants, T-shirts & sandals anywhere. People must remove their footwear before entering some of the temples.

People who live in SE Asia seem to be getting more liberal in their dress. Shorts are no longer seen just on tourists, for example, but they tend to be knee length. I'm sticking to long pants in temples. However, I'm also going to pack long shorts to wear other places the next time I visit there.

I recommend that you wear light-colored, lightweight clothing since it's very hot & humid there, even in their winter. Some stores in the US, like REI, sell travel pants that are very lightweight. I actually weighed each item of my clothes before packing them to ensure I had the lightest items possible. I only packed one carry-on piece of luggage (even for trips up to a month long) so I needed to make sure everything I packed was optimum.

Have fun! Val in CA