Hi BrightEye! Congratulations on your newest venture! I wish you enjoy the journey as well as the result
Your question is a bit complex. I say this because oracles, to me, can be almost anything in life. A flyer that the wind blew over your feet saying something relevant can be an oracle. A particular scene in a movie can give you powerful advice at a time you need it most. Words are also superfluous. You can use flowers, shells, cookies, the changes in weather, etc to "read" the world. So, freedom may be a double-edged knife. It usually emprisons me but it shouldn´t you
If I were to make an oracle, I would be true to my taste, to my artistic style and not make the messages too obvious. The richness of symbols is in the person reading them, not on the symbol itself. If you dig animals, go for it. If you prefer gore, do a more somber type of oracle. You KNOW you won´t be able to please ALL the readers. Taste is a very finicky thing.
So, in short, work on something you like, that you are good at, make it challenging but not so challenging that only YOU will be able to figure it out. PLEASE think a lot about the other practical stuff which also plays an important part in a reader´s choice: borders (or lack thereof), card stock, use of color (some will fade with wear). As to size, I believe it really depends on your art. Shadowscapes suffered tremendously because of resizing which was a shame. If you need cards to be bigger, go for it. There´s a public for everything. Good luck and keep us posted !!!