It is the default 150x150 in Settings > Media.
Now, Arras Theme has settings for this though with various sizes, here's what that page says:
Auto Thumbnails Check this to allow the theme to automatically retrieve the first attached image from the post as featured image when no image is specified.
If you have recently changed your layout or edited the thumbnail sizes, you will need to regenerate your thumbnails.
Single Post Thumbnail 465 Width 190 Height
Sidebar Widgets 36 Width 36 Height
Featured Slideshow 490 Width 225 Height
Tapestry: Node-Based 230 Width 130 Height
Tapestry: Quick Preview 115 Width 115 Height"
I have three of those sizes on my homepage alone. I do have it checked to make thumbnails from the image in the post if featured image is set -- I imported 425 posts and don't want to have to go through all and set a featured image.
I did not change any of these settings, all are defaults. I am willing to make the "tapestry, node-based" square as well if that will help my problem. However, the single post thumbnail and the Featured Slideshow need to stay at the proportions in the default.
Thanks again for taking the time to help me! I really appreciate it!