Ascorbic acid is said to be sensitive to oxidation by atmospheric oxygen, and this is probably even more true for the salt (due to the negative charge). So I guess you need to protect it from air.
Once it's dry it should be fine, though.
Concerning the quantities, you need to use parts in weight which are proportional to the molecular weight of the reagents and to their stoichiometry. Water of course doesn't really count, as long as you have enough to keep everything homogeneous.
In this case one equivalent of acid reacts with one of base, so look for the MW's of NaHCO3 and ascorbic acid, and that's it.
For drying the salt, I guess the method of choice would be freeze-drying (i.e. freezing the solution of the salt and then putting it into a high-vacuum chamber). Water is removed without melting, and the solid (maybe a hydrate?) is left.
If you don't have access to this equipment, you can probably find a water-miscible solvent in which sodium ascorbate is insoluble (isopropanol? ethanol? I'm just guessing), and just add it to your aqueous solution to precipitate the salt.
You may try and have a look at the Merck Index and/or at www.uspto.gov
(US patent office); a patent may exist which describes the manufacturing process of ascorbates.