The figure quoted in MB (75 MB or 150 MB) would be the amount of web space in megabytes that they give you for that particular account.
Each web page, image, etc, on your website has a size in bytes. The average size of the HTML for most web pages is around 10 KB (kilobytes), and the average size of images usually varies fro a few KB for small images, to 100+ KB for larger images. But you don't need huge images on your website, so you should always create a smaller sized copy for use on your website of any images that you take on your digital camera, for example.
75 MB should be adequate for most websites unless you have loads of images (assuming your images are no more than 100KB each in size, you'd be able to put around 750 of them in 75 MB of web space), or multimedia content such as video.
I haven't used Mr Site personally, although I have noticed issues with the HTML syntax that it generates when I've seen other people's sites that have been created using it. The HTML errors in the Mr Site websites that I have seen are usually handled gracefully by most web browsers, but usually cause several errors to be displayed when validating Mr Site web pages using the W3C validator,
The W3C Markup Validation Service - this could result in accessibility issues and is bad from a technical perspective in general.