No. They will reach 24" easily. At that size, the span of their maxillary barbels will be 3.5'+. The barbel span exceeding tank width is a no-no. The fish will be nervous, skittish, will damage itself from darting and bumping.
They also appreciate much more room for swimming.
V
victor448
can tell you much more. He is our hands-down L. pictus expert-in-residence and has kept them for decades in a variety of tanks and conditions. Going to 500 gal 8'x4' made a huge and positive difference in his pictus.