Q. We use in our minyan (when needed) a very small Sefer Torah, usually used in the house of avelim r’l. I noticed that due to the small size of the letters, the alef zeirah of parshas Vayikra is almost the same size as a regular alef. Can you read lechatchila from such a Sefer Torah, especially on this Shabbos?
How much smaller does the alef zeirah normally have to be? Is that a Halacha Lemoshe Misinai?

A. Mishna Berura (32: 1) quotes Magen Avrohom that the size of the letter daled of echad should be four times larger than a regular daled on the rest of the letters. He then adds that it may not be referring to the actual size of the letters of this particular writing, but any letters even the most minute possible. Mishna Berura concludes that as long as the daled is a bit larger than any other daled written on this parchnent, it is acceptable.
Horav Shlomo Miller’s Shlit’a opinion is that in regards to the alef zeira the same applies, and if it only a bit smaller than the normal one complies. Although these letters are part of the massores and therefore were given in Sinai, the lack in correct size does not disqualify the Sefer Torah.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a