These buttons measure 3/4" or 20 mm round. They have two holes in them for sewing onto your items. They are plain white and flat on the back as can be seen in the last picture.
Yes they are washable and dryer safe. Yes you may use these on items in your Etsy shop or elswehere.
My button and jewelry items are not meant for young children (under three) who might like to chew on them because, like any small item, they are a choking hazzard.