Here is super long smock made from the awesome durable Opalo fabric.
 
These smocks can also be ordered 40 cm or 80 cm longer than the regular smocks making them extra awesome. The regular extra length is 40 cm which makes the below knee length. 80 cm gets to the floor. If you something specific, just type in the custom text field (length from shoulder to the hem is a good input for me).
 
Available in all the Opalo colours. The 1000g dark olive is shown in the pictures.