Nodes automatically accomodate to their text when option "Dynamically Adjust Node Size to Label Size" on the Editor tab of the File > Preferences... dialog is enabled.
The node size is adjusted using the settings of the Fit Node to Label tool (from the Tools menu). This tool does not support a ratio, however, you can specify horizontal and vertical padding around the label text through the "Horizontal Space" and "Vertical Space" options, respectively.
