With the release of Pokémon Let’s Go and Sword/Shield, there are much bigger party icons for Pokémon now that there’s more screen space for them. Compare the gen6-7 Snorlax sprite to what you see in Let’s Go; much bigger and wouldn’t fit properly on the screen. This applies to our new friends from Galar, who are showing off their higher quality pixels! Would an optimized Pokémon party UI script be ideal, or would remaking these new sprites into smaller ones suffice?
I too faced this problem of icon size while inserting Melmetal in my game. But the people in the DS Style Gen VII and Beyond Sprite Repository are editing these large icons to fit the limits of Icon size in GBA games. I used the Melmetal Icon over there and got it to work with a little bit of upscaling. Maybe editing large icons might be better for those who don't want to go through the hassle of a new party UI or can't update to a version of Essentials which will have an optimised party screen. We did face a similar problem when 3D modelled Pokémon were introduced and we didn't move to Unity and ditch RPGXP when that happened so I don't think we should be worrying about this new icon format.
I will agree with Golisopod User, it is very likely that someone will remake the party icons for the new Pokemon in an older format. My project uses a size of 672x544 and it is a pain to rework all the GUIs, completely not worth it if your only goal is to use the larger style party icons introduced in Let's GO/Sword&Shield.