For a 18" cabinet and narrower, I would recommend a 6" handle.

For a cabinet wider than 18", I would use two 4" or two 6" handles.

Thank you Rebecca! I don't really like the look of 2 handles. Is it possible to simply use a longer handle instead or would it be bad for the drawers when I pull?

You can absolutely use a longer handle. For a 24"+ wide drawer, you might be able to get away with a single 8" handle. Any longer than that and they get much more expensive than purchasing two handles. Depending on the manufacturer, the long handles may be called "Appliance handles" and are more hefty than normal handles, so just pay attention to that in your research!

Do you have pictures of your cabinets/drawers and kitchen....that would get you the best answer.