You can use the Google XML Sitemaps plugin for wordpress. I've been using it for quite a while and its the best one out there and is completely automated after the initial stages of setting it up.
About your question :
I found this on the FAQ section of the plugin homepage. Since he mentions this, there may be a feature in the plugin where you can have multiple sitemaps, unsure though.What is the maximum size of a Google Sitemap?
According to the Google Sitemap FAQ, your sitemap can contain up to 50,000 URLs or reach a file size of 10MB (uncompressed!). However, I would recommend you to split such large sitemaps into various smaller ones which allows Google to retrieve only the latest ones regularly. This will save you a lot of traffic.