Hey there! I'm no developer but I can try to give some advice.
I suppose you could consider it two different ways. One is just look up the internal resolution of the gba. That would give you an idea what the map is, considering it's almost the entire screen. When i check wikipedia, it says 240x160.
https://en.m.wikipedia.org/wiki/Game_Boy_Advance
The other is to just take your image and look at the properties of the image and kinda, wing it from the resolution there.
Hopefully either way helps.