Rashi: The Yam's bottom, and its walls. (Refer to 7:26:2:1.)

Rashi: [At the top] it was thin and flattened out, like a cup from which one drinks, and illustrated with a flower and rose.

Radak citing Targum Yonasan: Like the brim of a oval cup gathered in a rose.

Rashi: It is 6,000 Sa'im, for a Bas is three Sa'im, i.e. an Eifah. The Yam held the Shi'ur of 150 Mikva'os of 40 Sa'im. One Mikveh is three cubic Amos (Pesachim 109b). The circular part was of diameter 10 (so its area was 75 square Amos), and it was two deep, so it was 150 cubic Amos. The square part was of side 10 (so its area was 100 square Amos), and it was three deep, so it was 300 cubic Amos. In all it was 450 cubic Amos, the Shi'ur of 150 Mikva'os.

Rashi citing Eruvin 14b: Here it discusses liquid measure. There it discusses dry measure ¹ ; an additional half of the liquid measure can be piled up above the walls of the Kli. Radak - this is true when the height is half the width. ²

Malbim: In the days of Ezra, who wrote Divrei ha'Yamim ³ , the measure Bas became a third smaller. We find that the weights increased; what was a half-Shekel became a third Shekel - (Nechemyah 10:33) "v'He'emadnu Aleinu Mitzvos Lases Aleinu Shelish ha'Shekel ba'Shanah" (even though the Torah obligates a half-Shekel)! "Lehaktin Eifah u'Lehagdil Shekel" (Amos 8:5, is a source to decrease measure of volumes and increase weights]. Yechezkel prophesied (45:11) that in the future, the Eifah and Bas will [again] be the same, and the Shekel will [again] be 20 Gerah. The Tashbatz (1:172) says like this.