Agree. If you do want to track down data, it is best to use the scientific name as the common names in this genus can be very confused.
Vallisneria americana may often be seen under the common names of Jungle Val and Freshwater Eel Grass as well as Giant Val, and is the largest plant in the genus; some sources give 3-4 feet for the leaf length, but others including Kasselmann (2003) give 2.3 metres (7 feet). There is a wide-leaf (blade 10-25mm wide with 5-9 veins) and a narrow-leaf (up to 10 mm with 3-5 veins) form. Some regions consider this an invasive plant; in some jurisdictions such as New Zealand it is illegal to propagate, sell or distribute this plant.
Valisneria spiralis is the other large species, referred to sometimes as Italian Val for reasons I cannot pinpoint. The leaves may be flat or twisted, and up to one metre (3 feet) long (Kasselman, 2003).