1: There is only one size. The highest resolution from Fotolia. If you are using them for web you will want to resize them with a graphic editor such as Fireworks or Photoshop or really any image editor. Having said that if you are building your site in Muse, you don't have to do anything. Just add the image and Muse should convert it for you.
2: Once they are licensed, they are yours to use indefinitely.
thank you for the reply... so, assuming that I use Muse for my Web Projects I would be able to download the resized images (by Muse) on my desktop. Having said that I could theoretically use Muse as an fast workaround to resize images on the fly and save it to my desktop afterwards, right?
Not sure how fast of workaround that would be. Would be way longer than just changing the DPI in Fireworks or Photoshop. To use Muse to convert it you would have to add it to a page, then export the HTML, locate the renamed file, etc.
Muse is great if you are using the Stock images within Muse site. As it will optimize it for your site. It would not be a good tool for reducing image sizes for other use in other apps/projects.