With the spell simulacrum you can create a half-real illusion of a creature with HD no greater than 2xCL and it has half the originals HD and abilities according to that.
Now dragons have age categories. Would a simulacrum of a gold great wyrm (30HD) have the abilities of a great wyrm at half strength so to say or would it have the abilities of a golden dragon with half the HD, in this case somewhat between juvenile (14HD) and young adult (16HD)?
The size should be that of a great wyrm, as you have a disguise check so observers think it is the real thing unless they beat your disguise result. But what about abilities that only the older dragons get? Like Divine Aid, Fire Aura, Luck,