If you're creating an N-Gage game using J2ME, you'll be using MIDP 1.0 (and Nokia's UI API for sound and full-screen graphics). MIDP 1.0 doesn't have any specific tile support, just the ability to draw rectangular images. So you're free to make the tiles whatever sizes you like.
N-Gage will also run C++ games written using the Series 60 1.0 APIs. You can get an SDK and emulator from Forum Nokia's Series 60 section
. I've not tried this, but I had a quick look at the documentation and again it seems to just support drawing rectangular (and masked transparent) images.
If you're a serious game developer, you can register as an Authorized N-Gage Developer
and get the N-Gage C++ SDK. I've no idea what extra features that contains.