Hi @positivfit,
I hope you are well today and thank you for your question.
What can I do to ensure a consistent slider size? Just keep choosing very large pictures?
Yes you should use slider images at least in size 1920px X 550px
Additionally, how do I control the size of the features image WITHIN the post?
You can control the size of the featured images by using the custom solution shared in the following reply.
https://colorlibsupport.com/t/i-want-to-change-the-size-of-the-featured-image/#post-50672
Best Regards,
Movin