What is a spinach head is equal to?

I would suggest that in this case what is described as a "head", is actually a "bunch". Spinach is typically sold in a bunch (or already prepared and bagged...or frozen), as it doesn't grow in "heads." Further, there is probably no need to be very precise for a ravioli filling recipe, and I am guessing this filling has other ingredients. You probably want to avoid frozen for the pasta filling. It could work, but is often very wet once thawed.