What is a "K" in this context? I dont know. But here is a clue from comments to the original article:

Celboy, good question. Barbara went back to Paul Franklin at Double Negative and asked him to run us through the exact numbers. Turns out the 5.6K files took up a little less space than he had remembered. Here are his notes: ### 5.6K: 5616x4096; A full 5.6K was actually about 100 meg for the exr and 122 meg for the cineon/dpx; 8K: 8192x6144; approximately 150 meg for the exr and 200 for the cineon/dpx; exr files are run length encoded whereas cineon/dpx is not, so as well as being more efficient they also vary in size depending on how much detail (difference between pixels) there is on a frame to frame basis; My mistake on the downrez figure for the 8K to 5.6

Looks like the K indicates number of lines per frame. They use a formual to determine number of pixels per line *4:3 ratio or something like that.