Image sizing is definitely a bit complex, especially with the use of high density displays and such, but I’d consider the biggest they’ll be shown on the page—regardless of the breakpoint—and make sure they’re all at least that big.
I took a closer look at the CMS setup you have, and these all appear to be grabbing a “thumbnail” version of the post image although I’m noticing that on your most recent addition (“The time and space of mornings”) you’re using the same size image for the “main” image as well:
Given you’re not really using a larger display of the image on the blog post collection page you can probably omit the second image field on the collection itself, but if you end up wanting to use a larger version then I’d recommend optimizing one version for the smaller list view and another larger version for the post view.
I’m not sure where I should be altering the height on all of them to make them all the same as on the size bar (Layout, Spacing section) it’s showing them all as 0. And if they’re different heights to start with, will this not mean they’re still uneven?
This is only the minimum height of the image (to allow it to shrink as needed) and keep in mind that your image shows the styles for the “Post Image” and not the parent container “Post Link In Images” which is where you’re ultimately going to want to control the height:
The styles on the “Post Image” element is going to be 100% height and width (so the image completely fills the available space of it’s container element) and then make sure you adjust the Fit property so that it doesn’t distort the image itself—instead allowing it to be cropped so it fills the entirety of the container:
Once you make those two changes to those elements you should see all images are aligned to the same height