Hi,
Thank you for the access.
However, I was not able to access the Fields management area. This requires a super administrator access, unless you change the default ACL settings of HikaShop.
I can see that you created a "specifications" field, and it was filled for the product "Milano Oak" and it does display its content under the "Specifications" tab of that product.
So I think that you're on the right track.
You don't need to call the custom field specifications. You can call it "height" and only enter the height of the product in it.
Then, create another custom field in the same way, and call it "width" and only enter the width of the product in it.
You'll then have two rows in the Specifications tab, one for the height and one for the width.
And like that, you can create as many fields as you want to have as many specifications as you want under the Specifications tab.