How big of a portal does one need to allow Ghasts to enter the Overworld?
According to the Wiki in 1.7.2 a Ghast could enter the overworld
1.7.2 - Ghasts can now enter the Overworld in survival with bigger Nether portals.
just how big does a Nether Portal need to be to allow Ghasts to enter the Overworld?
Solution 1:
5x5 should do the trick, at minimum.
If you can somehow manage to squeeze a Ghast into the portal the resualting effect shall send the Ghast to the Overworld. Even if the portal on the Overworld is smaller.
Spawn eggs were only used in the nether.