Note that the options to change font type and size are available for PDF files only, as shown:
Using a font type of Fixed is good for guitar tab and other files where the font alignment is important. Proportional fonts may be better for visibility if alignment is not as important.
To change the destination file type, select the blue button (.txt) and you will get the option to select .pdf or .txt:
If .txt is selected, the font type and size will be disabled. Plain text files cannot contain any formatting. Tip: save your charts as .txt for future editing, but also save a .pdf version with the font size and type set for maximum visibility. You can edit the TXT version, then re-save a new PDF.