Hello Jarret,
If you use same size of image ( original image ), the thumbnail will be also same size. If you upload different size of image the thumbnail will be also different size. You can hard crop the thumbnail image to make all thumbnail same size. For the hard crop you can use this plugin https://wordpress.org/plugins/acme-fix-images/ ,
The recommend size are as follows, but you can create image size as you want.
Set thumbnail size : 500 * 280
Set Medium size : 600 * 365
Set Large size : 840 * 480
Best Regards!