From A Wiki of Ice and Fire
Size relative to other swords
The bastard sword is mostly distinguished from a longsword because it has a grip long enough to allow two-handed use. Bastard swords have slightly longer blades as well, but they remain light enough to be wielded one-handed should the need arise, unlike the greatsword that requires two hands.
Appearances in the Novels and size speculation
The most prominent bastard sword seen in the novels is the Valyrian blade Longclaw, wielded by Jon Snow. Jon describes it as a good half foot longer than the longswords he was used to. Duck also wields a bastard sword on at least one occasion.