Size 10 crochet cotton (DMC) is the 'standard' size of this type of 'yarn'. The number is the diameter of the thread. The lower the number the 'thinner' the thread (I had to check my supply to see if I remembered this right, I always forget which way it goes). Hand embroidery thread is not crochet cotton. When I use size 10 I use a tapestry needle with a bigger eye, but you can use what ever needle that you have that the thread fits through the needle eye.
For me the size 10 for applique is a little thick. I always use 2 threads of the hand embroidery thread or the lighter weight crochet cotton. It is also know as 'tatting' cotton.
Good luck on you search!