Script to adjust width of dropdown

Use the custom validation script to detect the change on the dropdown that triggers the resizing.  

Here's an example.

var oFld = this.getField("NextDropdown");
switch(event.value)
{
    case "Select1":
       oFld.rect = rctSelect1;
       break;
    case "Select2":
       oFld.rect = rctSelect2;
       break;
}

In this example I use 2 preset rectangle arrays. The presets are better than trying to manage resizing dynamically. To get the preset rectangle values, just resize the dropdown to be whatever you want. The use the console window to capture the current rectangle size.