In the admin section, go to Products -> Add New Product
Fill in the product details for the main product, then save it.
once the product is created, go to the product update page for that product and click the button to add Options.
Since the options will effect price, select Advanced options.
Set the name of the option to Size, when prompted, add the default size.
Add additional options for the product to the list, then click finish to go back tho the product update page.
from the "Choose the variation of this product to edit..." List chose each option, make sure the "Only update <option name> options" box is checked, update the price and click the update button.