They are 3 to 10mm. For smaller stems i.e. less than 3mm I use the smaller silicone type tubes but I have trouble with those as I don't have good dexterity, so I lose a few by crushing the stems. I find it best to let the rootstock and scions get bigger so they are thicker and stand up to being handler by my banana fingers. I only lost three grafts out of 23 this year.