The obvious answer is that this story is about a diamond that is, literally, as big as the Ritz-Carlton Hotel. But lest we give Fitzgerald and his title short shrift, we should try to dig a little deeper.
First is the idea of exaggeration. We talk in "Writing Style" about the way that Fitzgerald exaggerates everything in this story. It's all about the hyperbole. And that's exactly what's going on in the title; it's not just a large diamond, but a huge, enormous diamond the size of a giant hotel. It's all about garish excess.
Second, comparing the diamond to the Ritz-Carlton Hotel connects the fantasy wealth of the story (a giant diamond) with the wealth of Fitzgerald's American readers (hotels, luxury cars, etc.). It brings in the satirical aspect of the story. In 1922, when Fitzgerald was writing, hotelier Cesar Ritz had recently died (in 1918), but his hotel legacy was gaining its ground. (Fitzgerald himself used to frequent the Ritz Hotel in Paris.) By using the name "Ritz" in the title, Fitzgerald brings to the front of his reader's mind the image of extravagantly wealthy Americans – the subject of the story's satire (see "Symbols, Imagery, Allegory" for a closer look). Incidentally, it's interesting that the benchmark of the Ritz-Carlton Hotel chain would turn out to be the Boston branch – thus an appropriate reference for young Percy – but not until 1927, several years after the story's publication. Fitzgerald's title is even more apt now than it was in 1922.