Q. On answer 900 you write that the different opinions on the dimension of a tefach are 11cm, 10.16cm, 10cm, 9cm and 8cm I distinctly remember seeing somewhere that there is an opinion that a tefach is less than 8cm. Is that true?

A. Although most Poskim set the shiur tefach according to Rav Chaim Noeh zt”l to 8.cm. There are indeed opinions that maintain the amount is less that 8.cm. (or even slightly more.) Simchas Yisroel (4: n. 247) quotes the shiur to be 7.84cm while Chazon Ish shiur varies between 9.82 and 9.42 cm., depending if the tefach has the fingers in a “sochakos” or loosely closed position or if we are dealing with fingers “atzuvos” or tightly held together. Midos Veshiurei Torah (5: 18) and Taharas Cohanim (p.5) quote an opinion of only 7.6cm. According to these opinions windows that are 55cm. wide would meet the seven tefachim minimum requirement.
However, Horav Shlomo Miller’s Shlit’a opinion is not to rely on the onset on such lenient opinions, when you can easily open two windows and have a complete shiur according to all.

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