Karima,
Cuttings (Dormant Cuttings) should be at least 3/8" in diameter (9.5 mm) and lignified with a minimum of 3 nodes.
One note from observations, initial handling and storage are very important...
The dormant cuttings should be stored in plastic bags with a desiccant, I use 2-3 tablespoons of shredded long Fibered Sphagnum moss, but any clean dry material (shredded newspaper, paper towel etc) should work. They then should be stored in the refrigerator asap.
If fresh dormant cuttings are place alone in a plastic bag and left at ambient temperatures for several hours, condensation will occur in the bags, from the sap. If they are then stored in the refrigerator, the chances of mold growth will be greatly increased when they are later warmed up to start rooting. This mold growth occurs very quickly once they are removed from refrigeration.
Good Luck.