Image size and texts of the slider

Hello,

I’m new to Twenty fourteen theme. I managed to activate the default slider in the header, with the aim of showing some featured posts when users access the blog’s homepage.

The problem is the size of the images displayed in the slider, that in my case do not cover the width of the slider section, seeing a big black space on the right. Should this featured images have special measurements to be shown in the slider? I leave my blog URL just in case it helps you to make a better idea of the problem: Lonifasiko

And apart from that, the post title and the tags shown in each post of the slider are too big, and the panel containing them is also too high, covering a big part of the featured image. How to make this texts smaller os has anything also to do with the size of the image?

Thanks very much in advance!