There are two ways to get the size guide displayed:
You can assign a size guide to one category or a few, by selecting a category on the size guide editing screen
Once size guide is assigned to the whole product category or categories you can still disable it for single products by checking option Hide size guide from this product
Or you can assign a size guide to a particular product by using the “Choose size guide” box on the product editing screen:
If you select a size guide for a particular product, it will be attached to that product even if the products category has its own size guide assigned.