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According to the pictures on the website, the bulb should be 6kg.
As to the length, the RS2000 is a scaled up RG65 not rs750,but the important thing to remember is when you scale up a dimension the area increases by the square of that and the volume increases by the cube of that (ie double the length and the volume increases by 8 times) so an rs750 bulb shape 573mm long would weigh around 24kg!
Using the RG65 bulb as your start and aiming for 6kg as shown on the website plans page the weight needs to increase around 8.6 times which gives a scaling factor of about 2.05x. so multiply the length of the RG65 bulb by 2.05 and you should be about right.
If you would rather use the RS750 shape multiply by 1.66 which gives aprox 357mm

Can understand your answer, and please tell me if I'm wrong (newby), but it's premise is based on the bulb weight of a RS1500 at 6kg. It would appear logical, that the weight and size of the bulb should be increased to accommodate the larger RS2000?

Ooops! yes the picture does show the RS1500 not the Rs2000 so it will be more than 6kg.
One way to find the correct weight would be to complete the hull and fin, add weight equivalent to the rig ,radio etc. then float the hull and add weight until it floats at the correct level and that gives you the necessary bulb weight.