Option A
https://apps.shopify.com/mechanic
+
https://tasks.mechanic.dev/sync-inventory-across-product-variants
Option B
https://apps.shopify.com/stocklink
Option C
https://apps.shopify.com/bundleup
Option D
(wait for @aosoble to finish his new app)
TLDR: These are all hacky ways to accomplish this, and there really isn’t a great way to handle having multiple variants sharing the same inventory count. What I would recommend is either: only offer one size/variant for the product, or else turn off inventory tracking entirely and then manually track how many you’ve sold … then turn the inventory back on and set it to 0, once you don’t want to sell anymore.