Objectively sizes would never be legal.
It would require outside involvement via the Mii Maker, and you can't really be too bothered to go through the process of ensuring a Mii that is the correct size that isn't 50/50 (guest sized) exists on the setup. It's far too much work than it is worth.
However Special Moves are a whole different ball game, and even then it's really just learning what the move can do and how to counter play versus it. Mii Brawler's Up Special 2 and 3 are both kill moves, and have the exact same set-ups (for the most part) but the stage and DI counter-play is different for both.
Additionally, a Mii Brawler with Down Special 2 has an incredible more amount of maneuverability and can recover high to center stage pretty consistently.
Mii Gunner's projectile game changes pretty drastically depending their special moves. A Gunner with Grenade may be very special happy and toss one out any chance they get, while the Charge Blast user will likely save it to get 23% off of one exchange.
Down Special changes a lot, you have to consider that they could have a reflector, magnet, or shield breaking tool (that also can cover landings / edge guard).
Swordfighter has access to a Cape, Counter, or Wizard's Foot in Down Special; similar to Gunner you need to be wary of what option you could throw at him.
A Swordfighter with Hero's Spin has an out of shield option that can kill, while a Swordfighter with Skyward Slash Dance may opt to dair gimp more since they can likely recover even if they go pretty deep.
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I can see the reasoning, but most players use Mii Fighters as a one-trick pony based on their Special toolkit. The mentality the player has is literally plastered on their Moveset. There will be more match-ups or situations you have to learn about, yeah, but as Mii Fighters are very unpopular anyways the only fighter you need to consider reading on really is Brawler, as he is objectively the strongest one (at least in Wii U). Even then, Brawler's optimal sets focus on 2X32/2X22 meaning you only need to be worried about what Up and Side Special they have since they would logistically be the least consistent factor. People say it would be more work than it actually would be because they're pretty lazy about it and probably wouldn't want to see Miis more prevalent in Smash based on character status.
It would require outside involvement via the Mii Maker, and you can't really be too bothered to go through the process of ensuring a Mii that is the correct size that isn't 50/50 (guest sized) exists on the setup. It's far too much work than it is worth.
However Special Moves are a whole different ball game, and even then it's really just learning what the move can do and how to counter play versus it. Mii Brawler's Up Special 2 and 3 are both kill moves, and have the exact same set-ups (for the most part) but the stage and DI counter-play is different for both.
Additionally, a Mii Brawler with Down Special 2 has an incredible more amount of maneuverability and can recover high to center stage pretty consistently.
Mii Gunner's projectile game changes pretty drastically depending their special moves. A Gunner with Grenade may be very special happy and toss one out any chance they get, while the Charge Blast user will likely save it to get 23% off of one exchange.
Down Special changes a lot, you have to consider that they could have a reflector, magnet, or shield breaking tool (that also can cover landings / edge guard).
Swordfighter has access to a Cape, Counter, or Wizard's Foot in Down Special; similar to Gunner you need to be wary of what option you could throw at him.
A Swordfighter with Hero's Spin has an out of shield option that can kill, while a Swordfighter with Skyward Slash Dance may opt to dair gimp more since they can likely recover even if they go pretty deep.
- - - - -
I can see the reasoning, but most players use Mii Fighters as a one-trick pony based on their Special toolkit. The mentality the player has is literally plastered on their Moveset. There will be more match-ups or situations you have to learn about, yeah, but as Mii Fighters are very unpopular anyways the only fighter you need to consider reading on really is Brawler, as he is objectively the strongest one (at least in Wii U). Even then, Brawler's optimal sets focus on 2X32/2X22 meaning you only need to be worried about what Up and Side Special they have since they would logistically be the least consistent factor. People say it would be more work than it actually would be because they're pretty lazy about it and probably wouldn't want to see Miis more prevalent in Smash based on character status.