I;m unsure about the different sizes. I know gnomes are the smallest, giants the largest and humans are the midpoint. But what about other races? Are dwarves for example closer to gnome size or human size in terms of Autumn Harvest? Is there any race but gnomes/svirfs that can bebefit from this as a legacy due to their natural size?

I would only suggest it on svirf or gnome, and then it would still be wise to often reduce for tough battles. Some info can be found at help size, http://www.carrionfields.net/helpsearch.php?id=1582

There are more than just three sizes.

1: Reduced gnome/svirf
2: Gnome/Svirf, I presume, goblin.
3: Human, Halfies, Dwarves, Duergar, Elvish races, etc.
4: Giants, Minotaurs, etc.
5: Enlarged class 4, or Dragons.

Autumn Harvest would work at it's peak with class 1 vs 5 but everyone
but class 5 has a chance for it to work to some degree. So, to answer
your question, Yes, but it isn't the most ideal choice.