The size of an annual border can be as long as you want but should only be as deep as your arms can reach.  You should be able to tend the flowers at the back without stepping on those in front.  If you can place the border where it can be reached from two sides, then it can be twice as wide.

A flower border with a house or fence behind it should have the tallest plants in the rear, working down in two or three stages to a low edger in front.  When the border stands where it can be viewed from two or more sides, the tallest flowers should be centered and scaled down to the edges.

Finally, proper spacing between plants is necessary for a full but uncrowded look.  Set mounded plants as far apart as their height at maturity; erect or linear growers, half the distance.