I ought to point out that even with a base Speed of 99, that means it still outruns Mimikyu, Hydreigon, Urshifu, Haxorus, Tapu Lele, Indeedee, Electivire, Arcanine, Krookodile, Landorus (Therian), and Rotom (Appliance).
Small Size Gourgeist is the second most popular size after Super Size. It all comes down to what you intend to do with Gourgeist, since one is a slow but bulky Grass-type, while the other one is a fast but frailer one. Super Size is good for outright attacking and on Trick Room teams, while Small, if you ask me, works better for support and inflicting negative conditions on opposing Pokémon, like Will-o-wisp.
I used a Small Size Gourgeist in Generation VII as a Leech Seed spreader for double battles, which worked better than I had expected.