About.com provides an excellent guide to mattress and sheet sizes in its home section. Similar information is also available at retailer websites, such as Lands' End and Bedding Experts.
About.com offers information on not only the size of flat sheets, but also on fitted sheets, comforters and standard mattress sizes. This is helpful in determining if bed sets are appropriate for a particular mattress. Lands' End and many other retailers offer sizing information specific to their products, which is an important tool in comparing standard and brand-specific sizing. Bedding Experts offers not only a mattress size chart, but a practical explanation of what the numbers mean.