Got a Rod of Lordly Might in a loot drop from a Red Dragon last night. When the first button is pressed (DMG p196) the rod becomes a flame tongue as a fiery blade sprouts from the end opposite the rod's flanged head.
But under Flame Tongue (DMG p170) it is a modifier for "any sword" and doesn't have a specific base sword. There is a Flame Blade spell (PHB p242) that acts like a scimitar but I don't think the RoLM is referencing that spell instead of the magic item.
So, what size sword does the RoLM make? A base long sword sized blade? Any sized sword? My character is a barbarian with the Great Weapon Master feat so having the RoLM become a greatsword and thus be "heavy" and enable the feat would be a big plus (as well as doing more damage). My DM has ruled that it can become any length I want but I was curious if there was official answer somewhere, or perhaps a reference in previous editions.
As a related question, can a RoLM count as a "heavy" weapon at all? None of the other weapons it can be; a mace, battleaxe, or spear are "heavy", so even if it made a greatsword sized Flame Tongue blade would it still count as a heavy weapon? Conversely, if I made it a scimitar sized blade could I then wield it as a finesse weapon?