I’ve done a couple of 20x20 tile based games. As was stated before, you have to implement your own tile system, so tile size is up to you.
If you want to draw 32x32 tiles on a 16x16 map, you would have to write to code to deal with the varying tile sizes. I recommend keeping them all the same size whatever you choose.
For speed, I draw my tile area to an offscreen bitmap that is one tile larger than the actual screen. When the user moves to the edge, I draw the old background onto a new offscreen bitmap (offset in the direction they moved), then fill in the edge. Since there can be a lot of tiles and J2ME is underpowered, you want to limit the amount of tile drawing.
When I render the screen I first draw the offscreen tilemap, then draw all the game elements on top of it. Some might argue that all the elements should be drawn to the offscreen as well, but in that case you would be redrawing all those tiles every time.
Wood