Not all sprite artists here commit to the Fire Emblem GBA style but they’re equally wonderful and amazing as well.
To start, the tools used to make sprites are many, to name a few common ones: Ms Paint or paint dot net, Piskel, Usenti, and Aseprite. Theyre all for free excluding aseprite, since its really specialized for pixel art but the tool doesnt matter much as long as you use what feels comfy or groovy for you to use.
The Pixel art medium can be learned in video or written tutorials, though to learn the Fire Emblem GBA style itself is more on written or you can reference the vanilla mugs.
Max dimensions, for the mug only itd be 96x80 but for the whole thing would be 128x112. Heres the format of it!
As a general tip, don’t get too discouraged when your first sprite doesn’t look like the ones you admire, not everyone started as pros so they all started as beginners and practiced til they became better at it.
Edit: I forgot to add something about the format:
the bluish area is where the mug should appear in the game, whilst the dark green areas are the cutoffs. the red and dark red areas are for the mouth and eye frames (bottom row mouths and side row eyes). The chibi is where the minimug of the character will be placed and is shown when you hover over them in the game