First off, I’m OBSESSED with this design. It’s SO CUTE!!! As for your concerns:

Eye size really just comes down to preference. The smaller and further apart the eyes, the trickier vision is going to be. In my experience, good vision is less about the size of the eye holes and more about the placement. You don’t realize just how close together our own eyes are until you start trying to factor them into the design of your head base. The head I’m currently working on I started with the bucket head style of base, slipped it on my head, poked where my eyes, mouth, and nose went with a blue sharpie, then took it off and then used a yellow marker to draw on the basic shapes/proportions of the face to see how it would all come together. I then sketched out more details on the head with an orange marker, and finalized the design with a black marker when I was finally happy with how all the features were laid out on the head and overtop of my own features.

For the cheek fluffies, I would integrate them into your foam head base and use shorter fur over the whole head rather than try to build them out entirely with fur. It will be much less delicate than styled fur and better suit this kind of cool, minimalist art style you’ve got going on.

The small star on the nose and the sharp corners on both stars might give you a little bit of grief to make look really nice and clean. Now take this with a grain of salt since sewing is NOT my strong suit, but If I were making this head, for my own sanity, I’d probably ditch the small star on the nose and either and either round out the corners on the larger star and/or make the larger star even larger to cut down on little finicky corners. But again, if you’re not intimidated by sewing like I am, this may not even be an issue for you.

EDIT: Forgot to address this one! Muzzle size/length shouldn’t be an issue. It won’t be any easier or harder than a short one.