If I make the images larger will they take up more space in the slideshow box?

Well, you actually can adjust the slideshow size, but the images themselves are a hardcoded value. What this means is that if you stretch the images too much, they will look pixelated and low quality. That said, here is some CSS you can try out to stretch out the images in your current setup (Bold News with an embedded slideshow) to be full width:

max-height: 500px !important;
}

body div.slideshow-window div.slideshow-slide img, .slideshow-slide-caption {
	width: 940px !important;
}

body div.slideshow-window div.slideshow-slide {
	width: auto !important;
	height: auto !important;
}

.slideshow-window {
	background-color: transparent;
	border: none;
}

.slideshow-controls {
	bottom: -60px;
}

Thanks again