The main gallery source file is found at Sources/Gallery.php inside your forum and this file can be downloaded to the computer and opened in a text editor such as notepad to edit.
The font size for the built image fonts range from 1 to 5.
The default font size is set to 3.
The higher the number the bigger the font (up to five).
Parts of the file to edit
Under the DoWaterMark function
Around line 4763
$text_height = imagefontheight(3);
$text_width = strlen ($modSettings['gallery_set_water_text']) * imagefontwidth(3);
The 3 in both of those lines is the font size which can be from 1 to 5
Around line 4778
// Write the text on the picuture
imagestring($image, 3, $xpos, $ypos, $modSettings['gallery_set_water_text'], $text_color);
The 3 in that function controls the image size and can be from 1 to 5
Going to be looking at using a different text watermarking system for Gallery Pro 1.3 seems you can use the imagettftext function to set the size of the text with pixels using truetype fonts.