The Mishnah in Kelim 17:10 says clearly that even according to Rebbi Meir, the Mizbe'ach ha'Zahav was measured in Amos of five Tefachim. (The only Gemara in Yoma that I can think of is on Yoma 16b, where the Gemara discusses the Amos of the Mizbe'ach ha'Nechoshes .)

However, the Sidrei Taharos on that Mishnah cites the following:

1. The Rash in Kelim cites a Tosefta as saying that "all Amos are 5 Tefach Amos, except for the Mizbe'ach ha'Zahav etc. which are 6 Tefach Amos." However, since the Rash doesn't even point out that it contradicts the Mishnah it clear that there is a Ta'os Sofer in the Rash, as all the commentaries there point out.

2. The Mirkeves ha'Mishneh, answering the question of the Kesef Mishneh in Beis ha'Bechirah 4:17 (why the Rambam didn't tell us what kind of Amah was used for the Mizbe'ach ha'Zahav) writes that according to the Rambam, the Pasuk in Yechezkel, which tells us that the Mizbe'ach ha'Zahav was 5 Tefachim, refers only to the Mizbe'ach ha'Zahav of the Bayis Shelishi. In the Bayis Sheni, the wood of the Mizbe'ach ha'Zahav was 5 Tefachim square, but the gold plating added another Tefach, making it 6 x 6 Tefachim!

The Sidrei Taharos, however, disproves this suggestion in a number of ways.

Be well,

Mordecai