Just so you don't end up disappointed:
1. If you specify 20" and 20oz, the length will probably be close, but the weight might be off. I don't think the techniques that the kamis use lend themselves to pinpointing the weight of a blade, especially since they are also trying to preserve the balance. It's different if you are ordering a stock blade (whether from HI or any other outfit) because they might have several on hand and can pick the one close to your preference.
2. The kamis usually make the handle girth proportional to the blade, since that's what seems right to them and matches the model specs. The Hanshee has a narrow blade, hence tends to have a narrow handle. Make sure to be very clear about what size handle you want for your large hands. If possible send them a wooden model of a handle and ask them to match it.
If the handle ends up too narrow for your hands, you can always put some kind of wrap on it, but that detracts from the appearance.