I'm not too keen on that particular colour scheme, but hey ho. So, here you can see the quite large wing area that the Mystic affords the RC pilot, and also the anhedral. Apparently, as the 'plane starts to fly, the wings load up and become "level" in the air. I'm not sure how that works when there is a tailplane between the booms up there, but anyway - I won't be including the anhedral.
Here is my first sizing sketch from 3D CAD :
I stretched the span slightly, and increased the chord of the wingtip a wee bit too (because I'm like that - always have to fiddle!) and a 3m-ish wingspan. This equates to 1287.2 sq inches / 9 sq feet (yes, jumping units here, you'll see why), and if I wanted 15oz sq ft ( this is why - I have no idea on loadings in deci-metre squared), the aircraft would have to be 8.4lbs in weight.
I think that's fairly do-able. A nice glider-ish power ratio would be 80W per lb (at the top-end - I want this to get away from a solo hand launch on the flat too) gives me just under 700 watts of power - easily achievable.
The next step then is to think of what that means in battery size and weight. I haven't got anything 4S lipo powered yet, but I feel this would be best here for a big prop pushing from where the jet exhaust area is. So 700 / 14.8 (I=P/V) gives me a current at WOT of 47amps. Nice.
Want it stay up long? Then let look at some BIG batteries. A Zippy 5800mAh battery comes in at 20 ounces of our 134oz AUW, and a decent 850kv motor comes in at 5oz. That leaves a lot of headroom for 8 servos, a flight battery, and the airframe.