This appears to be an issue with themes created with the Plazmic Theme builder. The designer can choose a font and font size but it is locked from modification by the end user.
I would suggest finding a theme that you like online and then locate the contact information for the developer. In my experience and dealings with people like JC from
JC Designs Blackberry Themes or Cherrichiodo (
All Things Berry - ... custom themes & wallpapers for your blackberry ) who is on this site quite regularly I find them to be very accommodating with reasonable requests.
If you emailed them and asked about having a specific theme modified to have a larger font I am confident they will be willing to help you out.