How much is Macbook pro battery capacity?

It would likely depend on which model Macbook Pro 15 inch you have. There are about 13 of them: http://support.apple.com/kb/HT4132

Also, check in the System Information (hold down Option key and select from Apple menu).

The Power or Battery info may provide the original capacity along with the current capacity.

After you read Barney-15E's advised source, you can download "coconut battery" from "macupdate.com".

Mine is below, and it show design capacity, current capacity, current stored charge.

Coconut battery may be your only choice as I'm just going from memory. I seem to remember finding that info, there, but I don't have a laptop anymore to check.