If you haven't created a Revit Family using connectors then this might not be interesting, till later. If you have then you're probably already nodding your head and thinking, "I know where this is going".
Revit Connectors for pipe (round duct too) offer us the ability to map their dimensions to a parameter so the user can adjust them easily in the project environment. It also allows Revit to resize pipe that is connected to a connector. Hmmm, I think I'm going to be using that word (connector) a lot.
Pipe sizes (duct too) are usually thought of using inches, such as 1-1/2" or 4" diameter. Those of you that work in the metric system are shaking your heads now. They told me back in fifth grade that we are going to change to the metric system. That was more than a couple years ago unfortunately.
Ironically we can't just create a radial dimension for diameter. It is technically possible to control the size of a circle with a aligned dimension (and Reference Planes etc.) for diameter but it isn't as obvious as the radial dimension tool. You'll find a lot of content therefore has a user parameter for diameter and then a calculated parameter for half the diameter to drive the radius of the host surface and its related connector size.
Another consideration is the unit that a user enters. It is quite handy to create a parameter using the sneaky little Associate Family Parameter button found in the Family Types dialog. If we use it we don't get a say in which kind of parameter is created, it defaults to Common (discipline) and Length (data type). If you want to let the user enter inches then you need to make the parameter in advance so you can choose the parameter from the list offered in the Associate Family Parameter dialog instead. When you make the parameter in advance you get to choose Piping (discipline) and Pipe Size (data type). This is a separate Project Unit that can be configured for inches, it is configured this way as a default setting in the Imperial templates.
Same thing happens if you use the Options Bar Label: Add Parameter approach. The only reliable way to get the discipline/datatype you really want is to make the parameter in advance. It would be quite nice if we could create a radial parameter for diameter an use it to manage connectors directly without the little "half-the-diameter" dance. Here's a Video to Watch.