The handle of a tool or product is a lot like the home page of a web site - they are both the user's primary touch point, allow access to the available functionality, and significantly influence how the overall product or site is perceived.
Another important similarity is that even though thousands of handles and home pages have been designed, there is always a challenge when creating a new one. This is understandable for home pages as there is so much variability in terms of content, information architecture and interaction design options. But it's surprising for handles, as they have been around (in both natural and man-made forms) for thousands of years - you would think we would have gotten it right by now.
In fact there are recognized guidelines for handle design - for example NIOSH's guide for hand tools includes recommendations on handle diameter, grip span, etc. But there is also great variability in handle design depending on the user population's range of physical characteristics, the particular task context (e.g. wearing gloves) and the product materials, to name a few. So any guidelines are going to be a compromise across a set of these characteristics. And note these metrics tend to focus on anthropometric fit, which does not always correlate directly with the user's perceived comfort.
It's important to keep up with changes in data and guidelines. One of the key dimensions in handle design is circumference. The NIOSH guide suggest a range of 1.25 to 2 inches (about 31.75 to 50.8mm). A new study published in the October issue of Human Factors has looked at this issue in more detail. Investigation of Grip Force, Normal Force, Contact Area, Hand Size and Handle Size for Cylindrical Handles is the very descriptively worded title. They take a detailed look at the how and why of finger anthropometrics and geometry impacting effective grip:
"For a small handle diameter,finger flexion results in skin folding and reduced contact with a handle. For a large handle diameter, the handle surface may not fit into the curvature of the finger because gripping flexes the fingertip."
At the end of the article, a rare find - a specific numeric recommendation is given: "the mean optimal handle diameter can be calculated to be 40mm".
Now before you go designing all handles with a 40mm diameter, keep in mind that this is optimized around a simple cylindrical handle - and its based off of US Air Force hand measurement data from 1971. Like all anthropometric guidelines, take with a grain of salt - use it as a starting point, but build rough models to evaluate fit with an appropriate range of representative users. In other words, the same user-centered design process that you would use to create any artifact - like a web site home page.