http://www.religionfacts.com/big_religion_chart.htm
Estimate of how many people are in each religion - Check the link above for complete details. Some interesting facts on the link.
Chinese Religion - 394 Million
Christianity - 2 Billion
Hinduism - 900 Million
Islam - 1.3 Billion
Judaism - 14 Million
Mormonism - 12.2 Million
Scientology - 70,000 to several Million depending on source
Sikhism - 23 Million