A relatively new macro that does not have much detail about the support it requires is the <%drawTagCloud()%> macro or Tag Cloud.

This Macro will not work unless you include the appropriate CSS class codes called by your theme template CSS file.

For example something like this:

.smallestTag {
 font-size: 0.8em;
}

.smallerTag  {
 font-size: 1.0em;
}

.smallTag {
 font-size: 1.2em;
}

.mediumTag {
 font-size: 1.4em;
}

.largeTag {
 font-size: 1.6em;
}

.largerTag {
 font-size: 1.8em;
}

.largestTag {
 font-size: 2.0em;
}

For more detail on this macro and the others available see this Macro Link.