The best way to ask if they have your size in Spanish if you are shopping abroad is the following?
-Tiene esta camisa de mi talla?
(Do you have this shirt in my size?)
You might also need the following sentences:
-Es muy caro/a, tiene alguno/a mas barato/a?(that's too expensive, do you have any cheaper ones?)
-Me lo llevo (I'll take it)
Vocabulary:
camisa: shirt
falda: skirt
pantalon: trousers
vaqueros: jeans
jersei: jumper
camiseta: t-shirt