If you're a small size (e.g. 8 or 10) buying clothes there won't be a problem. You'll be able to buy at market stalls, small shops and up-market places.
If you're bigger than that, you'll be limited to the more expensive places - department stores and boutiques. Marks and Spencers clothing ranges up to their big sizes (18+) but prices are high.... not 'Thailand prices' at all.
As for toiletries, standard sized items weigh a lot, so I take the bare minimum in mini sizes as I know I'll be out shopping next day and I'll be able to get what I want just about anywhere I go. Thailand, like other places in SE Asia, is awash with everything related to personal care, hygiene and beauty.
But buy your hair removal products at home because, apart from razors, it's hard to get what you need in Thailand.

Watsons (chain of chemists) is the place to go - they have stores all over the place. They stock many popular brands at prices much cheaper than at home. Look out for the fabulous moisturising hair treatments in plastic jars. And every 7-11 has fab minisized shampoo, conditioner and moisturiser.