As short as possible.

Unless you're 17 no one gives a shit about your GCSEs, nor whether you did a milk round when you were 15. Keep it utterly relevant, clear and concise.

Don't bother with a wanky 'personal statement' as they're clearly made up and are pretty much the same from CV to CV.

My recommendation (based on never once being turned down for a job, and from interviewing a lot of people) is one side of A4 max, with a good succinct covering letter. It's the letter that will 'sell' you and get you a job, not the CV.