Yay I finally solved this! However, I've only solved it using Prestashop v1.4. After hours or searching every files related - I found the file! Please go to your root folder then js folder > jquery folder > then look for jquery.fancybox-1.3.4.js find line 1105:
$.fn.fancybox.defaults = {
padding : 10,
margin : 40,
opacity : false,
modal : false,
cyclic : false,
scrolling : 'auto', // 'auto', 'yes' or 'no'
width : 560,
height : 340,
Then just change the width to whatever you want - I changed it to 695. Hope it works out for you!