tefillin

Does one put Tefillin on Chol Hamoad?

Is there a minimum or maximum size of Tefillin, and is there a difference between Shel Yad and Shel Rosh?

Why do we wear on Rosh Chodesh? Is it different than Chol Ha'moed?

For years, I had only seen tefillin retzuot blackened on one side. recently, a sofer suggested retzuot that are black on both sides. Which is better to buy?

Does one were Tefillin on Chol Hamoed?

May one try on tefillin at night, or does that fall under the category of not putting on tefillin at night?