If you're talking about a full chef's apron that ties at the neck and back then I would put the embroidery on the front chest (bib) part. If one of the aprons if for a busty lady you might consider where the placement of Bin and Vino are. Place the name high or lower, you don't want it to look like she is naming the "girls". Use a medium weight cut away stabilizer, it will hold the stitches better after multiple washings. The size of the letters depends is personal taste, how big do you want them, how they look and if there will be anything else embroidered on the apron.