Hi there,
There’s no optimal size for featured images. It can really be anything as long as you are satisfied with it.
Generally I prefer to use images that are big enough to fill the entire container so alignment is not an issue.
If your images are not big enough, you can align them this CSS depending what you need (default should be left):
.page-header-image-single {
text-align: center;
}
or
.page-header-image-single {
text-align: right;
}
Adding CSS: https://docs.generatepress.com/article/adding-css/
Let me know if this helps.