4mm is the most common size. Since green is a very useful color in this kind of needlework, have several shades available. Any fabric will work, as long as you can get a needle through it. If the fabric is dense, an awl can be used to make a starter hole. Size 13 chenille needles can also be used.
Knits are very stretchy, but can be used.
It can be stablized by using sew-in interfacing on the wrong side of the fabric. Gently hand wash finished projects in cold water with a mild soap. Hang or dry flat. To iron, put a terry cloth towel on your ironing board. Turn project face down and iron around the flowers.