Hi there,
Thanks for writing in! You should use same size images to get them in same look. You can also set a max height like below but remember if your image is very long in height then it might not look good rather stretched.
You can add this under Custom > CSS in the Customizer.
.blog .entry-thumb img {
max-height: 235px;
min-width: auto;
}
Hope this helps.
Cheers!