There is a setting in the blog settings for this, it also does depend on CSS which should be in your skin something like this:

.tag-cloud { list-style-type:none; margin:15px 0 3px -30px; }
.tag-cloud li { display:inline; list-style-type:none; }
.tagcount { font-size:x-small; }
.tag-cloud .weight1 { font-size:90%; }
.tag-cloud .weight2 { font-size:110%; }
.tag-cloud .weight3 { font-size:120%; }
.tag-cloud .weight4 { font-size:130%; }
.tag-cloud .weight5 { font-size:140%; }
.tag-cloud .weight6 { font-size:150%; }
.tag-cloud .weight7 { font-size:160%; }
.tag-cloud .weight8 { font-size:180%; }
.tag-cloud .weight9 { font-size:200%; }
.tag-cloud .weight10 { font-size:210%; }

Hope it helps,

Joe