Have the decals custom made to fit your space. The decals should take up 80 percent of your window space to really stand out. Anything smaller may be harder to see and not draw as much attention as you'd like.
Use a lot of color, and contrast colors for optimum benefit. White and light colors tend to be easier to see, but make sure the font is large and easy to read. Letters or designs that are too detailed or use a lot of cursive or scroll can be difficult to read. Capital letters are the simplest and will stand out the most.
Try etched decals for home use. These vinyl decals are made to look like the designs are etched into the glass and can look beautiful on patio windows or anywhere else.