Learn German: Brauchen Sie diese Größe?

Größe (size) is a feminine noun (die Größe). In this sentence, it is the direct object (the thing you need), which means it is in the accusative case. The word for 'this' before a feminine noun in the accusative case takes an -e ending, making it diese.

How would I ask this if I wanted to say 'Do you need THAT size?'

While diese technically means 'this', German speakers frequently use it for 'that' as well. If you really want to point to a specific size further away, you can use the definite article die (the) as a demonstrative pronoun: Brauchen Sie die Größe?

Why does the sentence start with the verb Brauchen?

To ask a yes/no question in German, you simply move the verb to the very first position. A normal statement would be Sie brauchen diese Größe (You need this size). By swapping the verb and the subject, it becomes a question.

Why is Sie capitalized in the middle of the sentence?

Capitalized Sie is the formal word for 'you'. You would use this when speaking to a customer, a shop assistant, or a stranger. If you were asking a friend or family member, you would use the informal du: Brauchst du diese Größe?