Thanks for bringing this up.
There are a few locations such as this where there are related issues - we are aware of this difficulty and that there are a few categories where an additional update on our end is required. Please note that for the required Size field if you populate the field for the specific that it currently shows as dependent on, you will be able to enter the size. So, as long as you have each of the required fields filled out, you should be able to post.
The secondary fix that Jacob mentioned earlier should resolve these issues. Our engineers are working on a solution for the remaining issues with Custom Item Specifics and we will update this thread as soon as more information becomes available.
Apologies for the continued inconvenience.