Implement stack using a linked list
This is generally good code, with standards-conforming names and style.

It appears that size is an afterthought variable. It is not scoped as being 'private', and it is not decreased in the pop() method, so it is buggy in two ways.

Additionally, even in the inner private Node class, I would recommend declaring the item and next as private, and also adding getters/setters for them.

Otherwise, this is good code.