How can I dynamically set the height of an iframe widget on someone elses site.

I am developing a widget that my customers will embed on their site.  The problem I have is the widget will vary in height and I cannot predetermine this height.  My first thought was to give them the iframe code to embed then some javascript to adjust the frames height when the page loads.  I found some other developers saying this would not work in all browsers.  I also think that javascript on one domain should not control something on another domain. (is that right?)  My current thought is to use ajax to load a page from my site into a div on their site.  Then use javascript on my site to adjust the size of the iframe. (the iframe would be on the page from my site)  I need the iframe to make the widget work.  Thanks for your thoughts on this.