Learn Portuguese (Portugal): Eles preferem o tamanho pequeno.

Preferem is the eles (they) form of the verb preferir (to prefer). In the present tense, regular verbs ending in -er and -ir always use the -em ending for 'they'.

Why are the words in the order tamanho pequeno?

In Portuguese, descriptive words (adjectives) almost always come after the thing they are describing (the noun). Instead of saying 'the small size' like in English, you literally say 'the size small' (o tamanho pequeno).

Based on this lesson's question words, how would I ask 'Which size do they prefer?'

You would use the word qual (which) to ask: Qual tamanho eles preferem? We use qual rather than como (how) or quanto (how much) because we are asking them to specify a choice from a set of options (like small, medium, or large).

How do I pronounce the -em at the end of preferem?

The -em at the end of Portuguese words is never pronounced like the English word 'them'. It is a nasal sound that sounds very close to the 'aing' in the English word 'faint' (without the 't'). Try saying 'pre-FEH-raing' through your nose!