Issues with correct feature slider size

Hello, I downloaded the Journal Pro Theme yesterday, and decided on the featured images for my slider on my home page. I can get the images up on the featured slider, but I cannot get the sizes to the compatible dimensions of 1920x800. I have downloaded Post thumbnail editor and some others but all will not let me choose the correct size and are therefore making my the image sliders larger upon first seeing my page. Is there any way I can fix that? Thank you!