Radak: These are the traits and Midos that Shlomo founded to build Beis Hashem. "Husad" means that he took counsel, like "Yisad ha'Melech" (Esther 1:8), "Yisadta Oz", "Nosdu Yachdav" (Tehilim 8:3, 2:2). Counsel in practice is like the foundation of a building. Shlomo took counsel for building the Beis ha'Mikdash from his father David, like it says above (Divrei ha'Yamim I, 28:11) "va'Yiten David li'Shlomo Veno Es Tavnis ha'Ulam."

Radak: They are the same Amos as the first Midah, to make the Mishkan. The Amah for Binyan (things attached to the ground) is six Tefachim; for Kelim, it is five Tefachim (Kelim 17:10).

Malbim #1: It is an Amah of six Tefachim, which is was used in the days of Moshe [for the Mishkan]. Stam Amos are of five Tefachim, for it says "ba'Amos Amah Amah va'Tofach" (Yechezkel 43:13).

Malbim #2: David measured the number of Amos when he dug the foundations. The verse teaches that Shlomo did not change from the Midah that his father measured.

'Rashi': Because it interrupted ¹ to say ba'Midah ha'Rishonah, it needed to repeat Amos. I.e. the length was 60 Amos, including the Devir and the Heichal.