Hi @y2kclubz,
Are your users consistently looking for the sizing chart information (is it something that the vast majority of them is doing on each visit)? Because popups are rather intrusive elements which can hinder a user's experience, if they are not absolutely necessary or offer useful information.
If that's not the case, then you could look at alternatives, such as having a discreet dropdown on your header or an announcement bar, or even a simpler one could be having a tabbed size chart where a user can switch to the system they prefer. The first option would require to save that information on the user's browser session (so they can browse the website and see the same sizing system on all products), a solution which is a bit more technically complicated, while the second one is easier to implement (but would mean that the user would need to switch to their desired tab on each product).
