Custom Featured Image Size?

Hi, if anyone could help me out with this problem I would be very appreciate. I’m a complete beginner I need help with the featured image size. I designed my custom theme in Artisteer and everything is perfect except for the the featured image size that I cannot figure out. On my homepage I am running a blog and when I post a featured image next to my article the size seems to be 50×50, however this is what I need to change to preferably a much larger size. Could someone please post a step by step guide on how to do this. Thank you