Blue, gold, opaline, platinum, lavender and cosby gouramies are all the exact same species - that is trichogaster trichopterus - or, if you preffer, they are reffered to as 'three-spot' gouramies. As such, the two fish you are talking about as if they are two seperate species are in fact just color morphs of the same.
It follows that they all reach the same size (5-6" with males being slightly larger than females), have the same temperament and have the same requirements and, indeed, this is the case.
How big are each of the fish at the moment? What sex are they? How long have you had them?
Generaly speaking, they should both end up the same size unless you have a male and female in which case the female will remain ever so slightly smaller (though she'd also be wider than the male when viewed from above, less 'streamlined' and more deep-bodied).
However, this species of gourami is one of the more aggressive of the common ones. Keeping two males together can have similar effects to keeping two male bettas together - you can imagine. Males will also chase females around - both of their own species and others - as well as harassing unrelated tankmates. Females aren't as bad but still capeable of harming each other if crowded and will deffinately take on tankmates that are invading 'their space'.
If your two fish are mature and appear to be getting along, they are probably both female. The size difference would be attributed to age rather than anything else. I would hope that you make a habit of watching your fish relatively often in order to observe any trouble - I would then assume that any aggressive behaviour would have already been spotted so I wouldn't think the problem is bullying when competing for food.
Having said that, there are other factors - such as the size tank they were in as juveniles and the environment they were in as fry - that could effect their rate of growth or even their potential size. You also need to keep in mind that inbred fish tend to be smaller than fish from a wide gene pool. Normaly you'd expect the blue to be larger than the gold if this were the case (as the blue is the 'wild' color while 'gold' is a selectively line-bred strain that arose as a mutation). Their health is also vital. A fish that's battleing a disease cannot use its energy on growing. That isn't to say your fish is ill right now - it could have been in the past. You also need to consider the simple variation that occurs between individuals. How many people do you know who are EXACTLY the same height as you?
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