In the context of this course, try not to stress too much on ensuring that everything is so proportionally perfect that it matches your reference to a tee. Matching the reference is far from our focus here - rather, the reference image is just a source of information that we draw from as we build up what is effectively a series of three dimensional puzzles, each one pushing our brain further in terms of understanding the world in which our drawings exist as being three dimensional.

I will admit though, the head construction stuff in the lesson is a bit scattered and split up right now - something I'll be correcting once my overhaul of the lesson content reaches that lesson. In general, you'll find the more up-to-date demonstrations on the informal demos page - that's stuff I've thrown together when critiquing some students' work, but which hasn't been integrated into the material yet, so you can think of it like a limbo area where valuable information/explanations remain until I can figure out how to best incorporate them into the main body.

To that point, I assume you're already aware of this but this head construction demo is the most up-to-date in that regard. To your question about how big the eye sockets they should be, err on the side of bigger rather than smaller. As to what shape, the specific upturned pentagon shown in that demo is what you want to aim for. In general, try to apply that demonstration as directly as you can to your animals. It won't always fit perfectly, but strive to apply its principles as closely as you can, with the eye socket shapes creating a wedge between them for the muzzle to fit into, and a flat edge across the top upon which the brow ridge/forehead can sit.