Hello C E Howard,
Your code above suggests you are using my .featured-gallery example. For those cards, you can change the width by changing the Bootstrap grid classes. So rather than use
<div class="col-xs-12 col-sm-6 featured-gallery">
which basically says, "make this card 100% on phones, and 50% on all screens larger," you could change it to this for example
<div class="col-xs-12 col-sm-6 col-md-4 featured-gallery">
This will give you a 100% width card on phones, 50% width card on tablets/small laptops, and 33% width card on large screens. The Bootstrap grid is based on a 12 column row, so each column-size (xs, sm, md, lg) needs to add up to 12 and this dictates width by screen-size, but you can set multiple column classes per card. You can find more information here: https://getbootstrap.com/docs/3.3/css/#grid-example-mixed-complete
The image will proportionately resize height based on the width of the Bootstrap column classes; therefore you do not set a height. I would strongly recommend you use a different type of card if you wish to set height, because if you set height on .featured-gallery cards, you will break the responsive layout and it will not look good on mobile devices.