You should be able to set maxWidth and maxHeight on the textfield.
Probably need javaobject to do it.
...
dim jo as JavaObject = theTextField
jo.runMethod*"setMaxWidth",array(100.0d)) ' set to maxWidth 100 pixels wide
jo.runMethod("setMaxHeight",array(20.0d)) ' set maxHeight to 20 pixels high
...
the values need to be Doubles - so 100.0d and 20.0d