How can I make all circle nodes in flowcharts have the same radius?

[PaulRBerg]

The circle nodes in my flowcharts have various radii. For example, see the "Sender" and the "Recipient" nodes in this diagram:

What theme setting should I use to make all circle nodes have the same radius?

Note: I would like the radius size to be global, i.e. apply to all circle nodes on my website, not just one diagram.