1. look at or consider someone/something carefully before forming a judgement or making a decision
size up the competition/situation
When you 'size something up' it means to look at something carefully and all of the information available before making a decision. When you 'size up' something it can also mean to make an estimate or a judgement about someone or a situation. Similar in meaning to 'weigh up' and 'make an assessment'. This is an informal phrasal verb used in social and professional contexts.