i don't mean to contradict any one but with out a doubt the largest P-stem goes to the Musa Ingens
"The undisputed King of the Musa family, this magnificent, wild species grows to a staggering 16 meters in height and literally towers over parts of the surrounding Nothofagus forest. It has been found growing on the edges of the highland Kau swamps at elevations of 2000 meters. This is the largest herb in the world. Pseudostems growing 2 meters or more in circumference, sparsely suckering, its leaves can reach a mature size of 5 meters long and 1 meter wide with pedicles growing to a length of 60 cm. The inflorescence is white with peduncle approximately 10 cm wide. Ingens has a heavily-fruiting raceme of 20 or more hands - each with approximately 16 to 25 fingers per hand. This plant requires a fairly drastic fluctuation of day to night temps of 30 degrees or more in order to thrive. It prefers cooler evenings.
here is a link to a picture sorry there vare rare and had to phtograph
http://contribute.chron.com/ver1.0/C...7b666.Full.jpg