Euros are better with dice!
I think you are on the right path in that you need to let the game determine the size of the tiles. It really depends on what is on the tiles and how the tiles are used. If it makes sense in the context of the game 3"x3" isn't too large.
I would let the tile size determine it's own size as you progress through the design process. I think the reason why Carc uses the size tile it does is that if you increased the size of the tile you would possibly run out of space while playing the game. if your playing area is contained then your tile sizes isn't as big of an issue considering you don't end up with a massive playing area
My playing area is very contained - all the tiles are available in the center of the table (meaning a 1" by 1 " area) and players need only enough space to place the tiles in a row.
On the other hand the game goes in real time so legibility and the ability to grab a tile of the table is very important, thus the 3 inch tiles so far.