Flooring Tile - Tile Sizes

We are in the process of retiling the breezeway to our house. The breezeway (including the closet) covers an area of ~80 square feet or roughly 12' x 9' and is separated from the tiled kitchen floor by a door and step up. Our goal is to to match as closely as possible the three-tone tiles in the adjoining kitchen so that the contrast between the two floors isn't quite the eyesore it is now. After a long search, the samples we've picked out are a pretty close match; at this point we're uncertain as to what size tile to order.

My question is one of aesthetics, probably. The current kitchen tiles are about 5 3/4" in size. Should we stick with tiles of a similiar size for the breezeway floor or, given its relatively small area, can we get away with using 8" or 12" tiles? Would the difference in tile sizes between the two rooms be a distraction? Also, the kitchen tiles are layed out in a diagonal configuration. How difficult is this to do for a novice like me who has never tiled a floor before. I understand the steps involved in laying out a horizontal pattern but nothing I've read so far gives step by step instructions on how the chalk lines are aligned and squared for a diagonal layout.

Thanks for your help!

Dannyy