1) what would be the most appropriate ground glass joint size for distilling small quantity of organic solvents (approx ~100mL) ? 19/26, 24/29?
I don't see how this is relevant. The joint doesn't affect the distillation path. Furthermore,joint size is only important for matching the separate pieces. Do you have a variety? Are you looking to purchase, and wondering what the difference is? Those sorts of answers are needed from you for us to give useful answers.
2) i often read that vigreux column are not efficient for Fractional distillation because of their limited theorical plates, what would you advise me to use instead ? a liebig condenser with glass raschig rings ?
All are equivalent choices if we don't have the specifics. What are you distilling (yes, I know xylene) what levels of what impurities do you want to remove,and to what standard do you want the purified product. FWIW, the Vigreux column is perfectly adequate for simple mixtures, and Raschig rings are only needed for specialized applications.
3) what would be the ideal length for the fractionating column knowing that i would like to distill small quantity of solvents (approx 100mL)
As above, it depends on what, specifically, you're doing. You're going to have to explain why,in high school, you're trying to make the 'bestest' solvents, instead of just purchasing to the needs of your final application.