Hi,
You need to add one additional value for each of your characteristic ( "choose size" and "choose color" ), then delete the variants of your product and the characteristics and add the characteristics back so that the variants are recreated automatically. Then, select these additional values as default values on the product edition page. Then, you can edit the variants using these two values and set their quantity to 0 so that the users can't buy the product without selecting both values.
If you don't want the values appended to the name of products, you can turn off the option "Append characteristics value to product name" in the config under the tab display.