First: Set the Max Send and Receive Size for the org to "unlimited"
https://learn.microsoft.com/en-us/powershell/module/exchange/set-transportconfig?view=exchange-ps
A valid value is a number up to 1.999999 gigabytes (2147482624 bytes) or the value unlimited.
Next: Create transport rules that block messages to and from your org if they are greater than 20MB. Why 20?
Base64 encoding increases the size of messages by approximately 33%, so specify a value that's 33% larger than the actual maximum message size that you want to enforce. For example, the value 64 MB results in a maximum message size of approximately 48 MB.
2 rules
One for inbound
One for outbound: