When installed properly a 1 inch tile is no more prone to coming loose than a 12 inch tile. People say that because if they are walked on (the tiles, not the people
) before they are grouted it is easier for a shoe to place enough leverage on the corner or edge of a smaller tile to release the bond. After they are grouted this leverage is non-existent. The smaller the tile the better grip your bare foot will have because of the grout lines. Smaller tile = more grout lines = more leverage.
A small tile will not leak more than a large tile. That's ludicrous.
Tile and grout are not waterproof. Your shower floor needs to be waterproof before you even touch a piece of tile. The wedi pan will do that. Whoever told you that does not know what they are talking about - or they are listening to erroneous information. Go ahead and laugh at 'em.