I have worked on a similar issue with AutoSize. What I have learnt is that this feature is by default ON for every new page you create in Visio 2010. This setting is saved in the page. So, if you turn it off for a page and save it, when you open the page next time, it will be turned off like you set it last time. [For pages saved by Visio 2007, since this feature was not there, the default status for Autosize is ON.]
That means, you have to turn it off for each page you create.