While there is no prescribed "official" size for mosaic tile, I tend to think of anything 1" or less as falling within that category. Travertine flooring tiles often hit the 1" mark--but rarely less (I have yet to see anything smaller on the retail market).
Compare Prices: Travertine Flooring Tiles - Mosaic
I chose this Pietra Art/Mexican series travertine tile from Florida Tile because it was so unique. Its diamond pattern, mounted on 12" x 12" mesh, stood above all other travertine mosaics I researched for this article. This would work beautifully as a travertine backsplash for a kitchen or bathroom