Sizing Photos for Home Page Slider

I’m having some issues getting the home page slider to display the image that I want it to in the correct dimension… I’m using your Social theme at the address:

Some of my posts I can set a featured image and it will show up full size in the slider, others it’s only a thumbnail. And it doesn’t seem to have anything to do with the size of the original image either since all of the images that I’m using are at least 2000x2000px or larger. If you go to my home page, you’ll see that the Barcelona image is tiny whereas most of the others are full size. Any suggestions on how to fix this? I also tried not having a featured image, but then the problem is that the slider picks a photo at random and again it’s not always the correct size.

Lastly, the slider only seems to work about half of the time in the Safari browser. It works fine in Firefox, but Safari will only show one image and no slider. SOMETIMES if I hit refresh, it will bring up the slider, but usually it will not.