Thanks to Home Support and Z-A's battle system taking Size into account, it's finally possible to see size difference for some Legends and Mythicals in a main game!
And of course we all have Hundreds of spares just waiting to be transferred into ZA to check sizes right? But if you don't, this video shows you some of it.
Unfortunately there really isn't a proper/consistent/easy way to Size-Hunt them.
The "better" option would be Go as it has way better size odds than main games, but you'll be waiting a long time for specific legends to be in raids, and then most mythicals and Zygarde are One Per Account encounters.
by Gaias_Minion