the TextSize function is not a script step. It can be used with Records>Replace Field Contents ("do not try this at home", i.e. be very careful with Replace), or within a script step like Set Field.
If you are not familiar with using calculations vs. writing scripts, I would highly recommend some basic filemaker training first.
FileMaker Pro 15 Help TextSize is indeed the function you need to use.
Set Field or Replace Field Contents are the script steps you need to run to calculate the new value.
Thanks a lot. This solved it.
Thanks for the advice, I am working on it.
These texts have a font size of 9 that I want to increase to 12. Manually I can do that by selecting the text and changing the font.
IMHO remove all formatting from text in fields, and apply the formatting through field objects on layouts
TextFormatRemove( fieldname )