First, I would decide how wide you would like the runner to be. Then chose a block size that divides evenly into that width. For example, if you want a 12" runner, then using squares that are cut at 2 1/2", 3 1/2" or 4 1/2" would work well. For a 15" runner, use 3 1/2" or 5 1/2" squares. For a 16" runner, use 2 1/2" or 4 1/2" squares. For an 18" runner, you could use 2 1/2", 3 1/2" or 6" squares. Keep in mind, that the finished size of the squares will be 1/2" less than the size you cut them at. Personally, for a table runner, I'd opt for the smaller squares, not the 5 1/2" or 6" squares. Have fun, and be sure to post a photo when it's finished.

This is so helpful, as I had been trying to figure out what size of blocks to cut for the width that I wanted. I have printed this and am saving this valuable information in my quilting file.

Lois gave you great measurements for various size runners. You can use squares, rails, strips, strings........ https://www.allpeoplequilt.com/quilt-patterns/table-runners/free-table-runner-patterns?slide=28eaf626-ebee-41df-8741-f6a66f1966fb#28eaf626-ebee-41df-8741-f6a66f1966fb