Why does it say that it was small?
Tanchuma (Emor 4): He grew in it 1 , and he was buried in it.
Radak: The custom was that only adults wear a robe, but not minors. She made a robe for him amidst her great love, and because he served Hashem. 2
Most explain simply, that it grew with him (she made only one; refer to 2:19:2:2). Sichos Musar (5732, 5) - why was this miracle needed? Are there not enough robes in the world?! She made it amidst love, tears, Tefilos and all her heart. With every stitch she prayed that he spread Torah. When he wore it, love of Yisrael and of Hashem entered him. Yefe To'ar (on Bereishis Rabah 95:1) - the verse says that she brought a robe every Mo'ed! Radak - it was only for the Mo'ed, in honor of the day. She took it back with her [until the next Mo'ed], for he was young, and prone to wear it on a Yom Chol. (Sichos Musar implies unlike this. - PF)
Malbim: The custom was for Nezirim and people who serve Hashem in Kedushah to wear a robe.
Why does it say "Ta'aseh" (future)?
Rashi: She did so [repeatedly, i.e.] once a year. (Refer to Bereishis 29:8:151:1.)
Radak #1: We often find like this (past in place of future).
Radak #2: There was only one robe, but as he grew, she adjusted it to be the proper size for him.
