How do I ensure that all my images on my website are the same size?

You can, but that is going to squish or stretch it based on the actual dimension of the image. You might want to be more specific when you say all images. Remember, even a Logo is an image but I am sure your intention is not to have literally each and every image on the site the same size.

img {
width: 100%;
}

If not resize them all with lightroom to the same size. You probably have to take web design track here to learn photoshop and how that works.

Use CSS to set it so that images don't lose their shape when resizing a browser window:

'''css img { width: 100%; } '''

Use HTML to stretch/shrink an image so they all have the same size (will have to be done for each image):

'''html <img src="img/numbers-01.jpg" width="100" height="100"> '''

Use image editing software to edit all images to the same size, i.e., Preview (Mac), Picture Manager (PC), Photoshop, Fireworks, or Illustrator.