Learn Spanish (Spain): ¿Tiene estos zapatos en el número cuarenta?

In Spain, you will often use the formal "you" (usted) when speaking to a shop assistant to be polite. Tiene is the usted form of the verb. If you were speaking informally to a friend, you would say tienes.

I learned that "size" is talla. Why does this sentence use número?

In Spanish, talla is used for clothing sizes (like shirts or jackets), but número is specifically used for shoe sizes.

The masculine word for "this" is este. Shouldn't "these" be "estes"?

That is a very common trap! The plural of the masculine word este (this) is actually estos (these), not "estes". That is why the sentence says estos zapatos.

Why do we say en el número instead of just en número like in the English "in size"?

In Spanish, when you refer to a specific number as a noun, you usually need the masculine definite article el before it. You are literally asking for the shoes in "the" number forty.