Contrary to the questioner’s claim, this is not a simple question. W ask what a film is but then specifies a film only equals feature films. We want to know what a feature film is, but the definition has varied considerably over time. And we want to know what the earliest feature is, when that distinction would depend on whether one uses the definition of “feature” in the early years of the 20th Century or a current definition.
The questioner mentions “Roundhay Garden Scene.” Its maker, Robert Le Prince seems to have been the first person to make a film as we would define it: flexible film running through a camera and recording a strip of images in real time. His surviving films were all shot in October of 1888, which is earlier than anything else I know of. “Roundhay Garden Scene” is one of those films, although as far as I know it only survives as photographs, not on film. His view of Leeds bridge and traffic is the only Le Prince film I’ve seen projected. It apparently ran about three seconds originally; the surviving bit is maybe three seconds. I don’t think there’s any record of the order in which Le Prince made his little films. I’ve never heard of any claims that anyone else made a film earlier than that, so Le Prince gets the credit, even though he never built a projector that would show his films.
“Feature” originally meant any film that could be “featured” in advertising. “The Great Train Robbery” would have been a feature, but it probably wasn’t longer than any of the other films on the program. Thus the first feature presumably was the first film whose title appeared on a poster outside a theater. We’ll never know what that was.
By the mid-1910s “feature” came to mean the multiple-reel film around which a program was built (with comic shorts, cartoons, etc.). While five reels was probably average from about 1915 to the end of the silent era, I don’t know how many reels a film had to be before it was considered a feature. Even today different groups have different standards (the AMPAS says 40 minutes, the SAG says 80 minutes).
Anyway, I think the answer that would suit the questioner’s presumed intention is that the film often claimed to be the world’s first feature film in the modern sense is “The Story of Ned Kelly” made and exhibited in 1906 in Australia. It was advertised at the time as being 4000 feet long and lasting over an hour. Since only a fragment of the film survives, there’s no way to verify those claims. At the standard projection speed of 16 frames per second, 4000 feet would last about 66 minutes. Not that projection speeds had been standardized by then, but at least the claims are plausible.
By the way, Leonard Maltin does not believe, as the questioner infers, that the two films mentioned are the earliest features. As you know, Leonard’s book only contains films that people can see on TV or home video, so earlier features that are not thus available aren’t there. I’m sure he’s aware that there were dozens, possibly hundreds of features made before 1914.