I found this blog article about export resizing options in Lightroom 5. It answered my beginner questions about export options like width & height, dimensions, megapixels, etc. so I'll pass it along.
Resizing Images in Lightroom 5 (LR5) The comments after the article are useful too.
Resizing Images in Lightroom 5 (LR5) The comments after the article are useful too.