in that case, you may use a selector excluding the li´s that have class="js-variant disabled"
You may use element attribute or just text. both will work.
selector: element attribute
{"_id":"forum_regex2","startUrl":["https://tr.uspoloassn.com/erkek-yesil-polo-yaka-t-shirt-basic-50249149-vr083/?integration_color=VR004"],"selectors":[{"delay":0,"id":"Description","multiple":false,"parentSelectors":["_root"],"regex":"","selector":"h1","type":"SelectorText"},{"delay":0,"id":"Price","multiple":false,"parentSelectors":["_root"],"regex":"","selector":".product__payment--price.hidden-xs ins","type":"SelectorText"},{"delay":0,"id":"SKU","multiple":false,"parentSelectors":["_root"],"regex":"","selector":"div.product-sku","type":"SelectorText"},{"delay":0,"extractAttribute":"data-value","id":"size-avail","multiple":true,"parentSelectors":["_root"],"selector":".js-product-sizes li > a:not(.disabled)","type":"SelectorElementAttribute"}]}
selector: text
{"_id":"forum_regex2","startUrl":["https://tr.uspoloassn.com/erkek-yesil-polo-yaka-t-shirt-basic-50249149-vr083/?integration_color=VR004"],"selectors":[{"delay":0,"id":"Description","multiple":false,"parentSelectors":["_root"],"regex":"","selector":"h1","type":"SelectorText"},{"delay":0,"id":"Price","multiple":false,"parentSelectors":["_root"],"regex":"","selector":".product__payment--price.hidden-xs ins","type":"SelectorText"},{"delay":0,"id":"SKU","multiple":false,"parentSelectors":["_root"],"regex":"","selector":"div.product-sku","type":"SelectorText"},{"delay":0,"id":"size-avail","multiple":true,"parentSelectors":["_root"],"regex":"","selector":".js-product-sizes li > a:not(.disabled)","type":"SelectorText"}]}