It is possible to change font size where size has not been specified by the web page designer - eg are you just getting v large text in the threads with the banners and headlines normal size?
Try looking at 'View' drop down menu then 'Text Size' - the default I think is medium.
Or look in 'Tools', 'Internet Options', 'Accessibility' and look at formatting. Normal setting seems to be all boxes unchecked.
Any help?