That is the exact right width for the part of the sidebar I went to put it on..... almost as if unseen forces had guided my re-design last month in anticipation.
Roo-- you're on wordpress.com, yeah? Click the share button, then "More" (button on the lower right of the embed options menu). On the second page, second column, there should be a Wordpress quick embed option. Just put in yer username and pass (this is completely secure, it's just going to be used once by the widget to auto-embed the code) and it'll do it for you.
This is why I plunked it down on the sidebar not in an entry. I didn't want the trauma of trying to coax it into behaving. Wordpress can be a little touchy about code in entries.