You don't really need an app if all you want to do is just display a size chart.
In can easily be done with a small modification to your theme.
Try posting a job in the Shopify Experts Marketplace.
The design of the size guide can then be created to your requirements and you won't have a monthly recurring cost.
You can maintain the content of the size guide(s) in a page using the page admin.
Asa @ Appifiny