Setting font size of textfield in livecycle designer using javascript

Hi there I got a fillable pdf created by livecycle designer. Spaces of most textfields are limited to the preset value (i.e., 150 characters). Can I use javascript to set the default font size to 12pt and if the input text exceeds the textfield area, the font size will be automatically set to "auto font size" but not less than 8pt. Is it possible?? I know I can set the font size to auto font size by setting the default font size to "0pt". But in my case, the default font must be 12pt and the minimum is 8pt. Is it possible??

Thank in advance