The Bug:
Even with its long arms that can reach up to 3.5 blocks, iron golems strangely cannot attack any mob that is 3 blocks tall
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Following that logic, players should only be able to reach 1 block away, but for a more balanced gameplay I'd say 2 to 2.5 blocks is good (Tools/weapons can increase that range).
This same glitch of the iron golem not reaching mobs being three blocks high (even though it can reach) also exists in Minecraft bedrock.