How much clearance and space does a helipad need for a helicopter to take off and land safely?

Federal Aviation Administration (FAA), U.S. Department of Transportation, has several advisory circulars available free of charge for heliport design. Check the FAA.gov site. Suggestions for the markers and markings can be found in the FAA publication titled "Heliport Design, Advisory Circular 150/5390-2A". See Link with some details and recommendations.