i'll just throw my 2 cents in here.

from the two corners the tiling should be as high as the bottom of the upper cabinets. assuming that you're going with something higher than 4"-6" high. i guess the main question is.......is the window centered on the sink? if it's not, you could bump the tile up higher in front of the sink, approximately the width of the sink base cabinet. perhaps with a different decorative mosaic tile. if the window is centered in front of the sink, which i assume it is, you could bump the tile up higher on both sides of the window and somewhat frame the window opening. i'm just trying to come up with an idea so you won't have a straight line of tile running the length of the wall at about 16"-18" across the entire wall. a picture would help alot.