Flippancy aside, do we know why it was not suitable? It isn't a headstone by its very nature. Is that the problem? I am not sure size has much to do with it really, graveyards are a hotch potch of sizes and designs. Perhaps the fact that it is wood, and so can be expected to degrade, may be the real issue.
Terribly sad, no matter what the reason, that there wasn't clear communication before it has become the issue it is. Communication is a two way thing, each side needs to be heard and hear. If it was the case that the family proposed this monument and was told it was not suitable but pushed on, then there is a different complexion to the matter. If there was no guidance and the monument erected, it is a different matter.
Either way, it is a very sad thing.