There is no problem with setting the featured post image size in css:
.archive .excerpt-thumb img {
width:300px;
height:150px;
}
And adjusting the heading and text also:
.archive .post .entry-title {
margin-left: 310px;
}
.archive .post .entry-content p {
margin-left: 310px;
}
But the featured image is cropped to 150×150 (standard wordpress thumbnail size) and you will have to alter the loop itself to pull the original image. Or change the thumbnail size to 300X150 in Settings/Media. FYI that will affect only images uploaded after changing setings.