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I guess there isn't any height limit but keep the majority players preference in mind. Some wouldn't like it when player just hides all the time...
The whole build minimum size should be 150 x 150 and maximum 250 x 250. However you make it is up to you but bear in mind that PvPers would want...
If you can't help it then feel free to do what Jag said :rolleyes:
Creative server please, that way we could inspect it in more details.