When I create a master page and assign a content page, the contentplaceholder in the content page is ridiculously small- about 2.5 cm across, and the text wraps to this size. Whereas in the master page, it fills most of the page; what am I doing wrong?

You can usually resize the content placeholder in design view. When you open the page in a browser, the content placeholder takes up whatever space is available, flowing to fill the available width. Have a look in Source view for width and height attributes on the placeholder, and remove (or adjust) these as required.

I think it may be because i am putting it into a table, how can I make the table a fixed size so that it does not squash the contents?