by the looks of the screen shot, you are using product.tpl...just put a if statement and in a javascript put
Code:
var selitem = document.getElementById('po'+whatevertheidforthe3.5x4selectboxis).value;
var qtybox = document.getElementById('product_avail');
qtybox.options.length = 0;
if (selitem == '3.5 x 4')
qtybox.option[0] = new Option('1000',1000); //populate the qty box with what you want
else
qtybox.option[0] = new Option('10',10);
granted, this only works on javascript users
as for the product options javascript mod, just an idea, it may be easier to just modify the form validation and putting it as an onchange to the select input...should require less modification and support importing/exporting the javascript