The operands of % undergo the usual arithmetic conversions to bring them to a common type, before the division is performed. If the operands were int and size_t, then the int is converted to size_t.
If size_t is 32-bit then -4 would become 4294967292 and then the result of the expression is 4294957292 % 7 which is actually 0.
If size_t is 64-bit then -4 would become 18,446,744,073,709,551,612 and the result of this % 7 is 5 which you saw.
So actually we can tell from this output that your system has 64-bit size_t.
