ok,
but I'd like to change the whole field size, setting only the font size.
It is the same behaviour offered by the standard input field (that is: when I change the font-size, the whole textfield changes)
try this,
http://jsfiddle.net/korenanzo/6hwwbptq/7/put 8 or 34 in the number field and see the different behaviours of the two objects
thanks,
RIc