char data[17] = { '0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','0','\0' }; // 16 zeroes + null terminator

strcpy( data, "abcdefgh");  // 8 chars written to data[0] - data[7]
strcpy( &data[8], "AA");    // 2 chars written to data[8] - data[9]

int foo = sizeof(f.data);  // 17
int bar = foo / sizeof(int); // 8

Serial.print(bar) // abcdefghAA

// Why bar == 8 ? it should be 10!