Alien structures are rather fickle, put the egg at least 24gu above the ground and make sure you give it plenty of head room too.
Another thing to note is that they tend not to want to spawn an an inclined surface, the OM is especially notorious for this.
Oh, and hopefully this is extremely obvious but the size of the egg model in editor is of coarse it's size before it grows. (Though this human buildables around this should be clear)