Not enough information and I can’t properly (and safely) answer your question. You can follow this link to Gama Tables - http://icpindexing.toddsit.com/documents/003046/gama%20vent%20tab.pdf
– and use one of the examples, I believe it will be “Example #5” on page 21 and table 6. I would also suggest going over the first few pages of this pdf for better understating of calculations.
You will need BTU rating of the gas water heater, its connector type (single or double wall), chimney total height, WH connector rise and lateral. All those values are required to properly calculate capacity of your chimney. Let me know if you need any help with calculations.