The traditional recipe for the base or shell is as follows. Shape the mixture to suit the circumstances, perhaps a little higher at the edges. For something larger, increase the recipe proportions. The rectangular one I often make uses double the above ingredients. Switch off the oven and allow the pavlova shell to cool in the oven.
Castor sugar is granular sugar that is rather finer than ordinary sugar.
However, ordinary sugar can be used quite successfully. Note that 8 oz sugar is about a cupful. In Australia, supermarkets carry pre-made pavlova shells. You can buy pavlova by the slice in various shops. Supermarkets also carry a mixture of sugar, dehydrated egg whites and things in an egg-shaped, plastic container, to which you just add a shell of water and beat to produce a pretty good pavlova shell.