d'oh. right. sorry did not look closely what we are doing here. and with the em the user can make a css rule for .tag_cloud li and set a basic font size. 

i think i would not multiply by 2 as this makes the number grow very fast. another question might be if linear scale is good or if things get too big between 1 and 20 em. but i guess we need to look at real examples to decide what is best.