You could use a VB script like this:
"<font fontsize='x'>" & left(fieldname,1) & "</font>" & substr(fieldname, 2, 9999)
You would replace the x with the font size. You would replace fieldname with the actual field name. The first letter will use the font size you set as the x value. The rest of the field would use the font size specified in the Format tab of the field.
Note: Do not copy and paste the VB script sample directly from your web browser into eDocBuilder, as the formatting used to display it in your browser may also be copied and may make your script work incorrectly. Please first paste the script into a plain text editor (such as Notepad). Then copy the script from the plain text editor and paste it into the Field Scripting tab of your field in eDocBuilder.