Hi,
Just to confirm, did you do the following when creating the expandable in GWD?
1. File->New File
2. Select 'Expandable' under Google Ads
3. Give the file name and hit 'Ok'.
At this point you should have an expandable ad with two pages: a 'Banner' page and an 'Expanded' page. If you look in the Events panel on the right, you'll see the first event handler is for the expand-button (this handler is executed when one clicks or taps anywhere on the Banner page). In terms of actual code, this function is called 'gwd.handleExpand_buttonAction' (which you can find if you go to Code view).
Now, to insert your custom JavaScript code, you have two options: you can edit the gwd.handleExpand_buttonAction function in code view, or you can create a new custom action for the expand-button. The steps for this second option are:
1. Right-click on the banner page and select 'Add Event'. This will bring up the events dialog with the target pre-selected as the expand-button tap area.
2. Under 'Event' in the events dialog, select Tap Area->Touch/Click.
3. Under 'Action', select Custom->Add custom action
4. Give your new custom action (i.e. function) a name and enter its code in the text box underneath the name. This is where you will add code to increase the size of the iframe.
5. Hit 'Save'. Now your custom action is registered as an event handler for the expand-button click event.