Resize feature image

The feature images on posts and pages are too big and I want to make them smaller.
I have found how to do so using css – see below – but the problem with this is that it has the same affect on the logo which like the feature images, resizes itself to 50% . Can anyone help to either (1) know css code which I could use to exclude the logo image from below or (2) know of a plugin for a logo. And (3) I also want to put a second logo on the right hand side of the header – I want to have two images, one on either side of the header. (4) I have tried to make a banner, which I normally do for these websites, but in this one I can’t seem to get it to accept the size I am creating even though it looks like it should. any help is welcome

.single .single-feat .single-thumb-full img {
width: auto;
// feature image parent class
.single-featured-image-header {
display: inline-block;
width:100%;
}
//css of feature image
element.style
}img {
height: 50%;
max-width: 50%;
}