Importing Variants

The documentation for the importing of variants shows
product_code,product_parent_id,size
main_product,,big
first_variant,main_product,big
second_variant,main_product,small

I have the following questions:
1-the characteristic "size" has to be all ready defined. Do the variants associated with "size" have to all ready be defined?
2-Why is "big" utilized on the second line where the product is defined? Could "small" have been used?
3-What do the third and fourth line do? Do they define new variants for the characteristic "size"? Do they define what variants should be shown for "main_product"?
4-If the characteristic and variants have previously been loaded, will this limit which variant is used. i.e. if you have big, small and regular, would this only load big & small.
5-when do you use the variant_id in an import?

I want a characteristic called "Size". Sometimes the product would have the "Size" value of Small, Medium, Large & Extra Large. For other products the "Size" might be 10,12,&14. How do I load the two different products and get the right variants? Currently when even if I import the third and fourth line that tells it which variant to use the product has all "Size" values loaded as variants.

Should I create two different characteristics both called "Size". One would have Small, Medium, Large & Extra Large. The second would have 10,12,& 14. But then on the import how would I tell the system which "Size" characteristic to use for creating variants?