Generally speaking, it’s a good idea to choose an exhaust pipe that is the same size the factory originally installed. If automotive math isn’t your thing, your other option is to check with the exhaust tubing manufacturer, or a trusted mechanic.
For street applications, your goal is for the exhaust system to have a flow path that’s as unobstructed as possible. This will allow the engine all the oxygen it needs, so it can run effectively and provide maximum possible power. For this reason, wider pipes are generally preferable. You can use this handy table to estimate the ideal pipe diameter for your own exhaust system.