Hi,
Adding extra field to "VM - Stackable Custom Field Plugin" is not possible, the official plugin doesn't have this ability.
Therefore, the 1st child option gets selected automatically. If the child product (that is associated with the 1st option) is out of stock, the "add to cart" button will be replaced by "notify" button. This is normal. But it might confuse shoppers at first to see the "notify" button there. They might not see the product attributes, or they won't attempt to change it once they see the "notify" button.
The plugin doesn't support adding extra "empty" value as a first option, but i would suggest you to create an another Child Product, and name it "-- Select Your Size --" pretend like, this is an ordinary child product (and make it's stock number higher than ZERO, so there won't be any notify button).
This is the only way for now to create "empty option" with a title of "Chose your Size", unless the developer of the plugin decide to add this feature.
(also; making the "add to cart" button un-clickable until the selection is made is a also a desired feature that we are hoping to see in that plugin soon)
Those are the improvements related to Virtuemart Component and the Stockable Custom Field.
Look here:
forum.virtuemart.net/index.php?topic=103067.0
it is a forum topic with 29 pages. The title of the topic is: "What bugs do you know of in stockable variants plugin", people have been discussing the bugs in that plugin for 29 pages ;)
Regards,
Support