Tag management is the new feature introduced in version 3.3 of Movable Type that I liked most.
However, the built-in automatic Tag Cloud sidebar simply grows too big for my site, because I have been using dozens of different tags to classify entries in my blog.
I found an easy way to limit the number of tags displayed in the standard Tag Cloud sidebar without editing any template at all, just changing a single line in the CSS stylesheet.
I am using the standard Vicksburg II (theme-vicksburg.css) style supplied with MT 3.3, but the same principle can be applied to any style.
Find the module-tagcloud section in the CSS file where style definitions for the Tag Cloud are located.
For the Vicksburg II style they are as follows:

.module-tagcloud .module-list {text-align: center; }
.module-tagcloud .module-list { list-style: none; }
.module-tagcloud .module-list-item { display: inline; }
.module-tagcloud li.taglevel1 { font-size: 19px; }
.module-tagcloud li.taglevel2 { font-size: 17px; }
.module-tagcloud li.taglevel3 { font-size: 15px; }
.module-tagcloud li.taglevel4 { font-size: 13px; }
.module-tagcloud li.taglevel5 { font-size: 11px; }
.module-tagcloud li.taglevel6 { font-size: 9px; }

Now, if you want to shorten your Tag Cloud by not showing the least important tags of your blog, all you have to do is to add the display: none property to the style definition for the least important level.
That is, change it to:

.module-tagcloud .module-list {text-align: center; }
.module-tagcloud .module-list { list-style: none; }
.module-tagcloud .module-list-item { display: inline; }
.module-tagcloud li.taglevel1 { font-size: 19px; }
.module-tagcloud li.taglevel2 { font-size: 17px; }
.module-tagcloud li.taglevel3 { font-size: 15px; }
.module-tagcloud li.taglevel4 { font-size: 13px; }
.module-tagcloud li.taglevel5 { font-size: 11px; }
.module-tagcloud li.taglevel6 { font-size: 9px; display: none; }

That’s all.
Now your Tag Cloud will be much smaller. If you want it smaller still, you could change higher levels of the .module-tagcloud definitions as well.
I hope this is helpful. Others suggestions are welcomed.
Stefano, Milan, Italy