The accents will vary in size depending on how warm they are grown and how much nitrogen (especially ammonical) they are fed. The 4-6" should be considered a 'minimum', a more likely height is 8-10" tall.

I would space them 10-12" apart on centers (e.g. planted every 10-12", the actual space between them may depend on how wide the plants are when put in the ground).

If planted too close, impatiens tend to strech and produce less flowers. you are much better off spacing them adequately and end up with lower growing plants with more flowers.

About 3-4 weeks before you put them out, pinch back all stems which will make them much more bushy. it takes roughly 2-3 weeks from pinching to flowering. When planting them, avoid giving them too high levels of fertilizer (especially nitrogen).

Starting the impatiens for a few weeks prior to planting them out usually result in flower initiation whereas feeding them heavily can result in delayed flowering, tall plants, and long stems with a few flowers at the end. (also applies to impatiens planted in potting soil to which slow release fertilizer has been incorporated).

FYI, our best performing plants were actually spaced 12-16" apart. In about two months they were touching and had to be pinched bag. Spacing them too close is generally a bad idea for impatiens.

Actually one little change...

the spacing between the rows should not be equal to half the distance between the plants. The correct spacing formular to use is this:
spacing between rows = spacing between plants x 0.886

example, if you space individual plants in a row 12" apart on center, the spacing between rows should be 12" x 0.886 = 10.6". If you space plants in a row 10" apart on center, the spacing between the rows should be 10" x 0.886 = 8.86" apart. Still make sure you stagger the plants from row to row so that the plants are not lined up exactly underneath each other.
apologize for the confusion.

Rgds
Kenneth

I have a question regarding this issue in dealing with volunteers from last year's bedding. I followed the spacing recommendations you've provided with my new plants, but all of a sudden, I had volunteers sprouting up between them! Now in some ways, this is terrific, but it makes it very difficult to get a consistent color scheme. I tilled the ground quite well before planting, but the volunteers came through (to my surprise!) quite well, anyway. Any suggestions, not only for this year, but in the future?