There are many, many small flowering plants that will grow where you live. Is the area sunny or shady, or both? What colour flowers would you like? Are you interested in annuals (live only one year but have a long blooming period) or periennials (live for years but have a shorter blooming period each year)?
I didn't have any luck in zone 6 with zinnias or the cosmos. One of my very favorite low growing plants in zone 6 was Bellis Perennis which came in red, pink and white mix...then I put white alyssum around it. (which is a very low growing plant). Lobelia in blue did well for me.
I grew all of these in the summers of zone 6. Another surprising one was Gazania that did well. And never forget the geraniums in lots of colors . Petunias did great in that same zone in the summers.
***edited to say I forgot the best of all. Iceland Poppies did beautifully from seed also. In fact, all kinds of poppies grew well from seed.
You can always just go to a retailer and try a mix of your own - tall, medium and short for the border. The photo below is just a bunch of pots that I bought and set together to see how the would look. Experimenting can be fun...
Oh, Dale...I love your picture of Bellis, Alyssum and Anemone...dreaming for me that next winter I will plant just what you have there...looks like you have the tall bellis...I always grow the 6" plant...would love to try that one
Dale, Austin is a lot colder than here. We use all these plants for our winters and plant in November. They do great throughout the winter here. Bellis perennes would die with out summer heat and sun. So...they have to be planted as annuals each fall and winter.
Mikel, one of the prettiest little gardens I planted was red, white and blue, way back in 1976 for the U.S. bi-centennial. The first row was white allysum, the second row was blue ageratum, and the third row was red button zinnias. The zinnias only grew 6-8". Burpee Seeds has a zinnia with mixed colors called Thumbelina, which grows to only 6" and is supposed to start blooming at 3". I think zinnias are so colorful.