Fisher Body is part of GM, of course, so their corporate archivist might be able to help. But I think it's going to be very tough for any of the others. Want ads in the local newspapers might give you an idea of the pay offered new workers, and you might occasionally find a newspaper story mentioning number of employees. Always check the company name on Google Books; you never know what trade journal might pop up. That may lead you to an annual issue of the trade magazine that listed the largest companies in the industry—probably by sales or output, but maybe by number of employees.

Detroit probably had an industrial directory that may give some indication of size. Standalone firms might have number of employees shown in a Dun & Bradstreet profile or securities directory. It's a real long-shot, but it's possible there was some sort of inventory of industrial resources taken by the city planning agency or chamber of commerce, or something done for the War Department. Trade unions for the appropriate industry might have a typed list buried in a box in a warehouse somewhere. Whatever Michigan office handled unemployment insurance would probably have received reports on the companies' payrolls—but it's unlikely that they've been kept or could be released to you.