I emailed Xanga directly several weeks ago about this because I wanted to know if I could limit my posted entries too.
This was their reply:
If you're on the Look and Feel system right now, then you'll be able to switch to Xanga Themes by going to your L&F page(http://edit.xanga.com/claf.aspx) and clicking Xanga Themes at the top.
Please note that once you make the switch (which is reversible, by the way -I'll explain later,) the coding you put into your Look and Feel won't affectyour public site. So, after you've selected Themes, you'll go to your public preview(www.xanga.com/yournamehere) and select "edit this page" at the top of yourpage. A window should pop out and you'll be able to change around yourtheme. And, because you asked, you can remix your theme - that is, take anexisting theme and change it so it's exactly how you like it, background andall.
You can also add and subtract modules as you see fit. At any rate, to change the number of posts, you need to roll your mouse overyour latest weblog entry and click on the yellow pencil icon to edit yoursettings. After clicking, you'll see your weblog settings and you can alterthe number of posts per page up to 20 or as low as 1.
So at the moment, you can only limit entries if you're using Xanga Themes.
I'm closing this topic now, PM me if you need it re-opening.