I think the size you're looking at should be right, to problem may be what save size the emulator expects.
For VBA, it should be something similar to this
(I'm saying similar because I'm aware there's different versions people use)
Once you're done, launch the game. Even if the save doesn't launch right, select "Import Battery File" (it is currently greyed out for me as I am not playing anything) then soft reset the emulator.
I don't use mGBA so can't help there, but the options should be something similar I reckon.