Hello,
You can edit js > prettyPhoto > js > jquery.prettyPhoto.js, just find this code
(function($){$.prettyPhoto={version:'3.1.3'};$.fn.prettyPhoto=function(pp_settings){pp_settings=jQuery.extend({animation_speed:'fast',slideshow:5000,autoplay_slideshow:false,opacity:0.80,show_title:true,allow_resize:true,default_width:500,default_height:344,counter_separator_label:'/',theme:'pp_default',horizontal_padding:20,hideflash:false,wmode:'opaque',autoplay:true,modal:false,deeplinking:true,overlay_gallery:true,keyboard_shortcuts:true,changepicturecallback:function(){},callback:function(){},ie6_fallback:true,markup:'<div class="pp_pic_holder"> \
Look for
default_width:500,default_height:344
Or you can choose not to resize your images at all by changing this line
allow_resize:true
Regards,
Ismael