Japanese tits

Hi everyone. New here but want to be ready for next year. I get a lot of Japanese tits in my persimmon tree. I would like to know the dimensions of a suitable nesting box including hole and perch. Preferably one that theaodaishō snakes can’t get in. 
also what is the minimum distance I can set a number of boxes apart.

Other birds I have that frequently visit are mejiro (warbling white-eye) a gorgeous little bird. Seems very fond of camellia flowers.  Does anyone know their nesting habits and is there a suitable nesting box I can make for them. 

many thanks in advance  

Bruce