Receiving a product that doesn't fit is pretty frustrating when you're a customer who bought online. Thanks to a custom sizing guide, you're able to help your clients to decide to purchase from you, and to choose the right size from the start, which limits the amounts of returns, and the costs associated with them. You can use both the mensuration of your customers (foot length or waist size) or comparisons with other brands (like a size 42 for Nike) to help your clients find the right fit within your catalog.
Once they found the ideal size, you can then display it as they browse the website, and you can also pre-select the correct variant when they land on product pages, which reduces by one click the distance they are from completing their purchase. It is also possible to let them buy the product straight away after finding their size if they started the process from the product page of something they liked, which also reduces the number of clicks needed to land into the checkout.
This custom development will respect your site's design, and all the data can be managed directly by your team with a simple spreadsheet so that you don't depend on anyone when managing this daily. Increase your sales, save money on logistics, and improve the satisfaction of your customers: everybody wins when they receive proper advice when they purchase from your store.