Use Edit->Text->Font Style->Size to change just the font size for selected text areas or text-paths. To change the current font size to apply to new text areas, use Settings->Styles and select the tab labelled Font.
When you specify a font size, FlowframTk guesses at the appropriate LaTeX font size declaration. The normal size is taken from the value given in the Document Settings dialog box (unless you have unchecked the Use relative font size declarations checkbox in the TeX/LaTeX Settings Dialog). This is used if you export your image as a LaTeX file. If FlowframTk has guessed incorrectly, you can select a different command from the combo box on the right, or you can edit it if you require a font size declaration that is not listed. Alternatively you can clear the value (by deleting the size declaration) to use the current document font size. Note that \veryHuge, \VeryHuge and \VERYHuge are not standard commands, but are defined in the a0poster class file. FlowframTk will only select these commands if the normal font size is 25pt. Remember that if you want to use very large sizes in your LaTeX document, you will need to use scalable fonts rather than the default Computer Modern.
Note that if you change the \normalsize value, you will need to reselect the font size for each of the text area's already present unless the checkbox marked Update all LaTeX font size declarations in current image is selected in the Document Settings dialog box.