The original (and the Color) GameBoy have a 160x144 screen, which is roughly 3" (without the ratio this number is completely useless anyway)
The GBA have a 240x160 screen.
So a 2.7" at 320x240 is more than acceptable for what the GameShell is supposed to do.
4" 800x600 (which I can’t find, the smallest I saw is a 5" one) start to get really big, and far from the original GameBoy form factor.
If I take Phoenix Display (which is not necessarily the most representative but yet they have a large panel of display) the, they only have 4.3" display at 480x273 to 480x800, which is not 4:3 screen and they are not going well with old consoles as they are 4:3 (or other weird ratio for portable one)