What is the ideal size for Tile images?
You
need tiles with the same or bigger size as the size of tiles in your
printed mosaic. If tiles in your printed mosaic will be 1 x 1 inch and
you will print it at 300DPI - you need your tiles to be no less than
300 x 300 pixels. You may use bigger tiles 400 x 400 or even 2000 x
2000 but not less than 300 x 300.
You
may just use large original 4 - 6 Megapixel photos. It will takes a
little longer time to save output mosaic in this case but - IMHO -
there should be not very long.
You
may downsample a very large photos to some middle sizes like 640x480 or
even 300 x 300, usually it is not necessary but in case of very large
libraries and mosaics it may speed up the process.
Also
you may prepare a cropped photos: cut one large photo into few smaller
different pictures. It's good to get a large number of different tiles.
See
here for more info.