This is an interesting idea... Perhaps, not a null value as that would be effectively '0' for integers (and some people may be wanting to create gadgets with no physical dimensions), but maybe specifying a negative height should set a standard height... Hmmmmm....
I guess that provided one assumes a MAXIMUM height for all gadgets (e.g. 20 pixels) and hence always leave plenty of space around each (e.g. from top of one to top of next = 24 pixels), then the actual size could vary within that. So I think such a scheme could work, provided you make it clear what these MAX & expected sizes should be. Ideally expose them as constants.