What ought to be maximum size Sitemap.xml for just about any e-commerece ASP .Internet website.

From Wikipedia:

The file should be UTF-8 encoded, and can't be a lot more than 10MB large or contain a lot more than 50,000 Web addresses, but could be compressed like a gzip file.

Well really if you're developing e-commerce website, it might be best practice to create your personal SiteMapProvider and keep links in database. So you don't have to be worried about any xml-based inadequacies.