2 sides.
No you don't need detail of responsibilities on every job you've had, unless you've only had very few jobs and they were all extremely relevant to the job you are applying for, which is unlikely.
Don't bother with school stuff, especially if it was long enough ago to be O Levels rather than GCSEs!
As a rule everything on your cv should in some way help you get the job you are applying for. No one is going to shortlist you for interview because you got some random O levels 20 years ago. You will be shortlisted because of your relevant skills and experience.
I'm going to find my standard cv tips, hang on..
Here you go, as pasted on several previous threads:
'Put personal information at the top (name, address, contact number only, not 'married 3 kids age 37 favourite colour blue').
Put a personal profile-type statement after that, just a couple of lines summing up what experience/skills you have to offer and the type of position/company you are looking for. Obviously make sure these tie in with the job you are applying for.
For each job, put dates, job title and employer, in reverse date order. For most recent/relevant jobs put a list of bullet points of main responsibilites and/or achievements. Do this with the job description for the job you want in front of you so you can emphasise relevant stuff.
Then qualifications/training. List in most recent order, include relevant training courses and higher education if you have it. Don't put Home Ec O Level. Everything on your cv should help you get the job you are looking for, and school exams usually won't unless you are a school leaver or very early in your career.
Don't put photos or anything else annoying and irrelevant, don't put it in a folder or on pink paper, don't staple it. It needs to be easy to read and easy to copy. Put page numbers and your name in the footer of each page in case of mishaps with photocopying.
It should be no more than 2 sides if at all possible. Don't leave gaps but jobs that were ages ago and/or are not relevant can be just listed with dates and little or no information about them.'