How long should a CV be?

Two pages, unless you have a reason. That is the whole answer for most people, and the reasons are narrower than you think.

One page

Fewer than about five years of work, a career change where most of the history is no longer relevant, or an application in a market that expects it — the US in particular, and some consulting and finance graduate schemes anywhere.

Two pages

Everyone else. Two pages is enough for fifteen years of work if you are disciplined about the early roles, and it is the length most recruiters in the UK and Europe expect to receive.

Three or more

Academic CVs, medical CVs and some senior public sector applications, where publications, grants or clinical work genuinely need listing. If you are not in one of those, three pages means the CV has not been edited yet.

What to cut, in order

1. The address. A city and country is all anyone needs, and a full postal address is a small privacy risk for no benefit.
2. Anything older than about fifteen years. Roll it into one line: "Earlier roles at X, Y and Z, 2004–2011."
3. Duties that repeat between roles. Say it once, in the most recent job.
4. Hobbies, unless one of them is genuinely relevant or unusual enough to be a conversation.
5. The "References available on request" line. Everyone knows.
6. School qualifications, once you have a degree or ten years of work.