The Sefas Emes asks why the Gemara uses the extra word "Aruchos," meaning "long," if it anyway states afterwards that their height was 11 Amos. He answers that the Beraisa is teaching us that even if they did not find 11-Amah-long Aravos which will lean against the Mizbe'ach, they should at least take long ones. This implies that they did indeed search for naturally grown 11-Amah Aravos, but if they did not find them, they did not cut them but rather took other long ones.

The Rambam does not mention the length of the Aravos. The Aruch la'Ner derives from this that the reason why the Mishnah mentions Motza as the source for Aravos is because only there they could find 11-Amah Aravos, but this is not an exclusive requirement, and if they found longer ones they would lean them below the Yesod, and if they found shorter ones they would place them on the Sovev, the upper ledge of the Mizbe'ach. Thus the Aruch la'Ner also believes that they did not cut them to this length but if they found them they would use specifically this length to lean against the Mizbe'ach.

Yoel Domb