As Bruno said, more insulation is better than less insulation. And for both health and refractory reasons, you're better off with satanite or some other refractory coating than leaving the wool bare. It will help reflect heat back into the forge and will also cut down on the # of loose fibres floating around.

Don't forget to take into consideration what your intended use it. If it's just razors and knives, then small is good. Why heat space you're not going to use?

But if you want to work on larger pieces, a new forge may be called for. But worry about that if/when the time comes.

And congrats on building the thing! You'll have a great time with it.