Variable length bitfield - c

Question

tl;dr: How can I properly generate a variable length bitfield in C?
I have a structure containing a variable number of elements for instance:
struct s{
int v1;
int v2;
/* ... */
int vn;
}
I'd like to create an anonymous bitfield of the size of the struct. Since i want low coupling between my two structs I do like this:
struct x{
unsigned long a:sz(struct s);
};
This code works but leads to issues:
What can I do if the struct has a size above 64 bits?
I can use several ints to store my bitfield but that gets messy and overkill.
I could also use a pointer to a void* but I suspect performance loss
By design my bitfield will generate AND for each operation over it deteriorate performances as seen in the following code
int main() {
560: 41 54 push %r12
}
__fortify_function int
printf (const char *__restrict __fmt, ...)
{
return __printf_chk (__USE_FORTIFY_LEVEL - 1, __fmt, __va_arg_pack ());
562: 4c 8d 25 cb 01 00 00 lea 0x1cb(%rip),%r12 # 734 <_IO_stdin_used+0x4>
569: 55 push %rbp
56a: bd 64 00 00 00 mov $0x64,%ebp
56f: 53 push %rbx
570: bb 04 00 00 00 mov $0x4,%ebx
575: 0f 1f 00 nopl (%rax)
struct x x = { .a = 3, };
for(int i=0;i <100; ++i){
++x.a;
printf("%u\n", x.a);
578: 0f b6 d3 movzbl %bl,%edx
57b: 31 c0 xor %eax,%eax
57d: 83 c3 01 add $0x1,%ebx
580: 4c 89 e6 mov %r12,%rsi
583: bf 01 00 00 00 mov $0x1,%edi
588: 83 e3 0f and $0xf,%ebx
58b: e8 b0 ff ff ff callq 540 <__printf_chk#plt>
for(int i=0;i <100; ++i){
590: 83 ed 01 sub $0x1,%ebp
593: 75 e3 jne 578 <main+0x18>
}
return 0;
}
595: 5b pop %rbx
596: 31 c0 xor %eax,%eax
598: 5d pop %rbp
599: 41 5c pop %r12
59b: c3 retq
59c: 0f 1f 40 00 nopl 0x0(%rax)
Ideally I'd like to do something similar to the following pseudo-code (but this is obviously invalid in C).
#define varint(sz) (sz > 32) ? \
((sz>64) ? "#error size too huge" : int64_t) : \
((sz>16) ? int32_t: int16_t)
varint(sizeof(s)/sizeof(int)) mybitfield = 0;
How can I properly generate a variable length bitfield in C?