I'm wondering about this type of exclamative. I know that certain types of exclamatives get accusative. Is it enough just to say "felem magnum!" or is something else necessary?
My first thought was something along the lines of Magnam Felem! (or Felem Magnam!).
For "What", you might be able to put Quid before Magnam Felem! but I'm not too sure about that though...
After quam, which translates literally to "how", you usually need a modifier (quam grandis feles! = "how big a cat!" = "what a big cat!"). "What a soldier!" would be qui miles! or qualis miles!