As with all Aroids, how big it can get will depend on how you grow it, how much pot space you give it, and the amount of care you give it.
In "wild" conditions these things can get tremendously big, somewhere in the neighbourhood of 3-4' tall and easily as big around when you measure the spread of the leaves.
The good news is that it's self-heading, which means you might have to provide it some stem support (particularly if you don't have ideal light conditions - it can get leggy) but you won't have to worry about providing it anything to climb. I'd try to get it as much light as you can, and the filtered light in your west window area is probably ideal (I've seen Philos burn in direct sun, especially indoors where the windows act a bit as focal lenses).
Here, then, are some questions for you:
What sort of soil/medium is it in?
Does the pot have ample drainage holes?
Are you open to the idea of repotting if it's in plain old soil?
Answers to these three questions will help me help you. Aroids are fascinating and rewarding plants to grow, if you pay attention to their (admittedly quite few and simple) needs.