Custom image sizes for post meta images

Greetings!

I have an interesting situation where I want to attach two images to a custom post type. One called Header and the other Thumb. I added a meta box with the two fields.

Later, I want to pull one of these, but since they’re not featured images I’m having an issue getting the url for the image with a thumbnail applied (e.g. example-150×150.png). The get_post_meta function returns the image url, so that’s all I have to work with.

Is there perhaps a better way to accomplish this? Or is there some function I’m missing that allows me to, like get_the_post_thumbnail fuction, apply a thumbnail to an image?

Any help is much appreciated! Thanks!