Animation can only be exported at 4:3 or 16:9 aspet ratio. This means that if your SU window is different from these ratios, there will always be some cropping.
I see you are on Windows. First of all, I would recommend this plugin to hide all your toolbars (before they get messed up when you resize your SU window):
http://forums.sketchucation.com/viewtopic.php?t=42926
Then this little freebie to resize your SU window so that the inner (model) space is exactl 4:3 or 16:9 (you will need to experiment a bit - try to export a 2D image and set it to export at screen resolution to see what it is):
http://www.brianapps.net/sizer/
It works like shown below; you right click on the edge of a window and tell it what size it should be. As you can see, presets can be saved.
4-3.png