What is the XPath expression to find only the first occurrence?

The correct answer (note the brackets):

(//span[@class='Big'])[1]

The following expression is wrong in the general case:

//span[@class='Big'][1]

because it selects every span element in the document, that satisfies the condition in the first predicate, and that is the first such child of its parent -- there can be many such elements in an XML document and all of them will be selected.

For more detailed explanation see: https://stackoverflow.com/a/5818966/36305

Dimitre Novatchev's answer is correct if you are expecting the class attribute to be equal to Big (without any other classes attached to the element):

(//span[@class="Big"])[1]

... which is similar to the following JavaScript expression:

document.querySelectorAll('span[class="Big"]')[0]

On the other hand, if you are expecting Big to be one of any number of classes in the class attribute (rather than the only class), you can use the following expression:

(//span[contains(concat(" ", normalize-space(@class), " "), " Big ")])[1]

... which is similar to the following JavaScript expression:

document.querySelectorAll('span.Big')[0]