Good luck with your project purestrain, looks really nice.
About bigger tiles.. Last night I tried to implement 3 levels of zoom, one with 8x8 tiles, 16x16 tiles and 32x32 tiles. The problem is that there just isn't enough memory for the 32x32 tiles. J2ME is very limited in memory and cpu power, so resizing them on the fly isn't an option. On my phone (Nokia EM 5130) the tiles as shown in the pic above are - in my opinion, just right.
Currently I'm implementing save/load, so we'll see how that turns out.
Thank you all for the feedback that's about enough I need for now. Thanks again!