As far as I know, the HP interface does not offer the granular level of control over the appearance that you seek. To achieve your goals you'll need to put on your programmers hat.
One way you could adjust the size of individual text boxes would be to generate an html file for the Hot Potatoes quiz and then adjust the CSS styles on the individual text boxes to make them the size you want. Then, upload the HTML file to use as the source file for a HotPot/TaskChain activity.
Another way, would be to use the jcl file as the source file for the HotPot/TaskChain activity, but then specify a configuration file that includes the CSS styles you want. The advantage of this method is that you don't need to regenerate the HTML and manually edit it every time you make an adjustment to the quiz. Also, you may well be able to use the configuration file on several HP quizzes.
Both of these methods require a working knowledge of HTML and CSS, and as such may be considered too "technical" by some teachers to make them suitable solutions.