With her small size; I need to give a surprise without going wrong.

You could always get her a gift card. Or, take her shopping and be happy and supportive while she shops. Also, things like capes, shawls, scarfs, jewelry are one-size-fits-all and, I am sure, would be appreciated by your girl friend. Womens' sizes vary between brands and styles, not like mens' clothes which are the same size between brands and styles. So, it is not easy to just pick out a certain size for a woman. I would suggest taking her shopping as I am sure, she would appreciate the experience of having you go with her to shop. Then, afterwards, take her out to lunch or dinner.

Steal her cloths and look at the tag. That should give you a good idea of her size. If you do get it wrong, keep the recipet so she can exchange it for the right size.