What is the maximum length I can use for a pringles/coffee can?
In general terms, the the longer a waveguide antenna is, the greater its gain and the tighter its beam. However, a long antenna with a tighter beam is tougher to aim and keep aimed. Don't use a can over a foot (305mm) long. Finding a longer can may take some time. Sometimes specialty gifts are shipped in a long tin can.
As a rule of thumb, once you get past a ratio of 3:1 for length to diameter you reach a point of diminishing returns. 2:1 is a better ratio for 2.4GHz.
A shorter can with a wider diameter is what you want. A can with a diameter between 73mm/2.87in and 94mm/3.7in would work best. 100mm/4in diameter cans (1-pound coffee can) are common and are the ones I recommend.