Question

Are there any size limitations for C structures?

Answer 1

From the C standard:

5.2.4.1 Translation limits

1 The implementation shall be able to translate and execute at least
one program that contains at least one instance of every one of the
following limits:

…
— 65535 bytes in an object (in a hosted environment only)
…
— 1023 members in a single structure or union
…
— 63 levels of nested structure or union definitions in a single
struct-declaration-list
…
13) Implementations should avoid imposing fixed translation limits
whenever possible.

Other than that, the upper bound is SIZE_MAX (maximum value for size_t).