Let's take a simpler example. Let's say you have a array int test[10]
which is stored like this at address 1000:
1|2|3|4|5|6|7|8|9|10
The complier knows that, for example, an int is 4 bytes. The array access formula is this:
baseaddr + sizeof(type) * index
The size of a struct is just the sum of the sizes of its elements plus any padding added by the compiler. So the size of this struct:
struct test {
int i;
char c;
}
Might be 5. It also might not be, because of padding.
As for your last question, very shortly (this is very complicated) the MMU uses the page table to translate the virtual address to a physical address, which is then requested, which if it's in cache, is returned, and otherwise is fetched from main memory.