One of the style options of nodes is the font used for the writing within them. The net has a default font & size for nodes, which may be changed by making sure no nodes are selected, and then choosing Style → Font (or by right-clicking on the net background and choosing Modify → Default Node Style → Font). A dialog box will appear showing the current font and size, which you can modify accordingly.
If you select some nodes before doing this operation, the new font will apply to the selected nodes only, and will override the default.
If you change the default font, and some nodes don’t change, it is because they have an overriding font. To reset them to default, select them, right-click on the selection, choose Style → Font and set them to the same font as the default.
Node Size: Whenever you choose a new node font, or a new font size, the nodes will be resized so that the writing within them fits nicely. In fact, the way you enlarge or shrink the nodes of a net is to choose a larger or smaller font size for them.
Links: To change the font for the labels of all disconnected links, use Style → Links → Link Font, or alternately, right-click on the background and choose Modify → Default Link Style. More Info
Character Sets: Keep in mind
that if your net must display international
character sets, then you must choose a Unicode
font that includes the desired characters. "Arial Unicode MS"
is usually a safe choice.