the product data should be entered directly in mysql.
Yes it is possible to have one select list that will control which of the size select lists is displayed, but you will need to hand code javascript top show or hide the select lists based on the onchange event of the parent select lists, eCart will not code this for you.
you could set up the checkout to allow processing manually or through paypal.
This will require some manual coding again to accomplish.
your manual transactions will need to be completed over the phone though, since it is illegal to store credit card information in the database.