enlarge featured picture on homepage

When i first installed this awesome theme, the featured images on my main page used to be very big.

Now all of a sudden they show up smaller.way smaller.

All I did was switching off a widget, but this should not be related?
I never changed anything regarding featured images before.

The weird thing is, i can not even set other featured images to my posts anymore!? any suggestions on how to enlarge the featured images on main-homepage?

thanks so much for the kind help !