Just bought two Dieffenbachia plants for my screened porch.
Some questions:
How big will they grow?

Maybe 2 to 3 feet, you are not looking for height so much as luxurious
growth of leaves. With insufficient light they will grow taller and less
leafy trying to get to the light. If your porch is cold they will likely
die.

Do they require much light?

Strong light maybe a little filtered sun.

Water?

Yes, mainly in the growing season but they are not too fussy

I have a curious cat - are the leaves poisonous?

Yes

David