Is there a max viable size for RewriteMap hash files?

I know this isn't strictly WPMU, but I need to solve it to migrate into WP. I think RewriteMap is the ideal solution for rewriting about 200,000 URLs I'm going to be moving on my site, and it looks like the hash map approach might be the best way to prevent possible dorky scripting errors.

But can the hash (dbm) handle really big lists (like 200K URLs)? Will there be too big a slow-down or too much memory usage?