.    .             .  .
 Hilchos Tefillin 32 (page 88) ????? ???? ?? ??? ??? ?????? ?? ???? ?"?
 The 'Size of a Small Letter' The Length of the Legs of the Letters The Left Leg of the Heh
 The 'size of a small letter'  There is a halachic notion of the 'os ketana' (size of a small letter) that pertains to many areas of stam. A 'small letter' means a properly formed yud, including its left point (see Mishnas Sofrim siman 36, os yud). In the context of small or medium lettering, the size of the yud should be commensurate with the size of the rest of the writing. There is a discussion amongst the poskim as to whether large lettering requires an equally large yud or a medium-sized one. The poskim do not define what constitutes "large" or "medium" lettering. (???? ??, ?"? ??, ????"? ?"? ????; ??????? ??????? ????, 77)
 The length of the legs of the letters The following are examples of where the measurement of a 'small letter' is used: the left leg of the heh, the left base of the tav, the left leg of the kuf, the base of the lamed, and the legs of the letters daled, ches, reish,  the straight (or long) chaf, peh and tzadi. All of these should be the 'size of a small letter.' Likewise, if the leg of any letter cracks, the remaining top part must be the size of a 'small letter' in order to be kosher. If the upper section is less than a 'small letter,' the os is pasul even if a child can read it correctly. (If the child cannot read it correctly, it is pasul -- even if it is long enough.) (???? ??, ?"? ??-??, ????"? ?"? ???; ??????? ??????? ????, 70-74)
 The left leg of the heh The poskim discuss the size of the left leg of the heh. The halachic decision is that it must match the os ketana. This is the size that would have to remain for the letter to be kosher even if the leg was partly erased or a hole formed nearby. According to some poskim, the os ketana mentioned in this context is different than above, and the smallest yud would suffice even for large lettering. The Mishna Berura's language, however, does not seem to support that. (???? ??, ?"? ??-?, ????"? ?"? ????; ??????? ??????? ????, 67)

 There is a rule known as mukaf gevil which mandates that every letter of stam must be completely surrounded by parchment. Chazal derived this halacha from the word 'u'chesavtom," which can be divided into two words - u'chesav and tom-i.e. complete writing. In addition to a hole adjacent to a letter, a hole within a letter can also disqualify stam. A hole large enough to be visible invalidates the stam even if it is completely surrounded by ink. According to most poskim, bedieved the rule only applies to the exterior, inked area of the letter, and not to the white interior. The Yerushalmi, however, extends the problem to the interiors also. Using a child to help identify a letter Which doubts can a child assist with? Permitted repairs based on a child's reading
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