I have not raised this species and have found no actual information on the size N. ventricosa gets except for 6 inches being about the average size of their pitchers. The information just indicates that N. ventricosa is a compact plant like N. sanguinea. I have seen reports of N. sanguinea growing 9 foot vines, so I would suppose N. ventricosa would grow similarly. Some Nepenthes can grow 3 or more times that size. In any event, if it gets too big, just clip a foot of vine off at a time and pot them up with some rooting hormone after notching the cut end a few times. A cutting can result in a new plant in a couple of months if all goes well.