There are seven map sizes in 0 A.D.:

1. tiny (2 × 64 = 128 tiles)
2. small (3 × 64 = 192 tiles)
3. medium (4 × 64 = 256 tiles); default
4. normal (5 × 64 = 320 tiles)
5. large (6 × 64 = 384 tiles)
6. very large (7 × 64 = 448 tiles)
7. giant (8 × 64 = 512 tiles)

Now from a linguistic point of view, it's a bit odd that the third option is called “medium” and the fourth “normal”.
The fourth option is both the mean and the median of all map sizes; therefore “Medium” would be the correct name (medium (adjective) means “About halfway between two extremes of size or another quality; average.”).
The third option is the default map size; therefore “Normal” would be the correct name (normal (adjective) means “Conforming to a standard; usual, typical, or expected.”).
This patch thus simply swaps the “Medium” and “Normal” names.

Ought to be uncontroversial.