There are two Flaming Swords of Rhuin, as part of a tradition with the Army Books and RPG. The RPG basically took the Army Book Battle Magic spells, downscaled them, and maintained their name to add them to the RPG. Wheras Flaming Sword is a Battle Magic spell that makes an entire unit's weapons glow with flame, the RPG spell does this:
"A fiery sword materializes in your grasp. It counts as a magic weapon with the Impact quality and Damage 4. You also gain +1 to your Attacks Characteristic for the duration of this spell, but only when using the flaming sword of Rhuin. The spell remains in effect for a number of rounds equal to your Magic Characteristic. You can retain it with a successful Will Power Test each round thereafter." Realms of Sorcery 2E Page 148
Typical Fire Mage stuff. It's not noted as being particularly large, just a sword of flame. It's a pretty good weapon, but it has a very short duration in the RAW.