Are the NTSC GameCube discs the same size as the PAL GameCube discs

If you mean physical size - disc diameter, etc. Then yes. There is no difference, just the way the picture is displayed and, possibly, in some cases, minor content adjustments. The discs are generic worldwide.

Matsushita (2.000 MB/s-3.125 MB/s) CAV mini-DVD-like 8 cm optical disk. Average access time: 128 ms; Capacity: 1.5 GB.

The Nintendo GameCube Game Disc is the medium for the Nintendo GameCube, created by Matsushita. Chosen to prevent unauthorized copying and to avoid licensing fees to the DVD Consortium, it is Nintendo's first non-cartridge storage method for any system released in America.