What is the recommended size and limit of URL's in the XML Sitemap? can we rename it?

Based on Google Webmaster FAQ: the uncompressed version of the site map may contains up to 50,000 URLs and a maximum size of 10MBs.
Renaming a submitted site-map will impact search engine crawler that tries to access an updated version of your site map.
Usually, failure attempts to download the site map are indicated in the site map section of your web master tool account.

As indicated, a sitemap should have no more than 50,000 URLs and must be no larger than 10MB.
However, if you have a website with huge number of pages, you can create multiple sitemaps then create a sitemap index includes all of them: http://support.google.com/webmasters/bin/answer.py?hl=en&answer=71453 As for renaming your sitemap, it won't affect the already indexed pages, just make sure to update Google Webmasters tool with the new name/link.

AS Webmasters Tools recognitions the uncompressed version of the site map may contains up to 50,000 URLs and a maximum size of 10MBs

Some browsers and servers may have limits, but those limits are well beyond anything we’d consider sane by SEO or usability standards.
For example, IE8 can support a URL of up to 2,083 characters.