Are there any known impacts to changing the maximum message size of the broker (e.g. from 1MB to 2MB)? Is memory allocation dynamic, such that this change would only affect larger messages. Are there any fixed size allocations based on the message size or anything else that could cause performance issues for the majority of messages that would be smaller than the current default.
The default setting:
No, there are no impacts as the allocation is dynamic.
That value simply limits the largest message size that can be sent by a client. If a client exceeds that message size, the broker will disconnect the client immediately as a means to prevent flooding, etc.