I would first suggest cleaning up your style sheets.

Pages that appear to jump around are usually caused by stylesheet elements being defined more than once.
First style sheet loads and layout renders, second stylesheet loads and layout changes - things jump.

And yes the text size widget can contribute to this ....  base style sheet has one text size ... but the text size widget default is different size.
First question is always - WHY do you want the text size widget - it does cause potential layout issues.  Its kind of a wank feature at best and im always willing to bet that most users never touch it anyway.

Select a fixed font size for your site - and be done with it - it the user has bad eyesight - they can use the features of their browser much more effectively to scale your site than what the text widget will ever do --- oh and to be honest the same goes for those wide/full/narrow page widgets as well.

Just my two cents ... your mileage may vary.

Westa