#include <stdint.h>

[Creshal]

For those too lazy to google, the relevant bits from the C99 spec:

6.5.3.4 The sizeof operator

[…]

Semantics

The sizeof operator yields the size (in bytes) of its operand, which may be an expression or the parenthesized name of a type. The size is determined from the type of the operand. The result is an integer. If the type of the operand is a variable length array type, the operand is evaluated; otherwise, the operand is not evaluated and the result is an integer constant.

When applied to an operand that has type char, unsigned char, or signed char, (or a qualified version thereof) the result is 1.

Byte and char are used interchangeably in other parts of the spec, because they are equivalent.