The flag of French Guiana is not an official national flag, as it is an overseas department and region of France. However, a popular unofficial flag exists, which features two equal vertical stripes and a large star.

Colors:

1. Green (Pantone 356 C; Hex #008752)
2. Yellow (Pantone 116 C; Hex #FFCD00)

Aspect ratio: 2:3

Geometric properties and measurements:

1. Two equal vertical stripes occupy the left half of the flag: the left stripe is green, while the right stripe is yellow.
2. A horizontal red stripe (Pantone 186 C; Hex #E4002B) divides these two vertical stripes, with a width equal to one-third of the height of the flag.
3. The right half of the flag features a black star (Pantone Black C; Hex #000000) on a white field. The black star has five points and is inscribed in an invisible circle with a diameter equal to half of the flag's height.

Please note that this flag is not internationally recognized and doesn't have exact specifications for its design elements. The Pantone colors provided are approximate and may vary depending on the source.