I don't think so. You have 3 options: Tile in X, Y, or Individual Tiles. Only the last of these (it actually is first on the selection list) allows both dimensions to be specified. All tiles will then be those dimensions.
However, if you choose either of the X or Y feed-throuh options, you can make the tile size an arbitrary size that is less than your machine bed, but one which will extend the last tile over the end of the bed. This would have the effect of making the last tile smaller than the others. I suppose the same would be true for the Individual Tiles option, but I haven't tried that.