Rationale

• in a previous PR I introduced a design-system.js constants file, which contained a SIZES constant.
• in a PR that predated that one, I added the ColorIndicator component with its own variety of sizes for its size prop.
• This rectifies the size system to use the xs->xl type sizes.
• To support that I added entries into the size map for xs and xl and transformed small medium and large into their two-letter abbreviated counterparts. The size values for those items are unchanged.
• I also added indicatorSize and labelProps props to the network-display component, which is required for the new add-ethereum-chain request UI