Magic animal questions

One of my players has a magical animal companion. I decided to go ahead and work out the actual stats (I usually wing this sort of thing). I'm having trouble understanding how to do the basic stats. ROP:M pg32 seems to indicate that I should go to ouside sources for the mundane stats of the animal and 'shoot' for those stats during the 'spending' for stats. Furthermore, it isn't clear how to deal with the size modifications to stats during the 'spending' process. What am I missing.

Can someone clarify the process?

Thanks in advance.

House of Hermes: Mystery cults has a full section on how to make mundane animal stats and effects of size (I believe after spending stats, add size x2 to strength and subtract size from quickness so size -2 is -4 str and +2 quick, Size +3 is +6 str, -3 quick).

There is also the book of mundane beasts at end of RoP: Magic to show you a bunch of mundane animals as base. Start with making the base animal and then make it magical.