Referencing a non static array list in a static environment?

Your code should look like this, especially your Student class using java encapsulation

public class Student {

   private String studentForename;
   private String studentSurname;
   private int courseworkMark;
   private int testMark;

    public Student(String studentForename, String studentSurname, int courseworkMark, int testMark) {
        this.studentForename = studentForename;
        this.studentSurname = studentSurname;
        this.courseworkMark = courseworkMark;
        this.testMark = testMark;
    }   
    public void setStudentForename(String studentForename) {
        this.studentForename = studentForename;
    }
    public String getStudentSurname() {
        return studentSurname;
    }
    public void setStudentSurname(String studentSurname) {
        this.studentSurname = studentSurname;
    }
    public String getStudentForename() {
        return studentForename;
    }  
    public double averageMark(){
        return (this.courseworkMark + this.testMark)/2;
    }
    public String grabFullName(){
        return studentForename + " " + studentSurname;
    }
}

And then testing via your Main class :

public class Main {    
    public static void main(String []args) {
        ArrayList<Student> studentList = new ArrayList<>();
        studentList.add(new Student("Adam", "Goldsmith", 70, 50));
        studentList.add(new Student("John", "Smith", 20, 40));
        studentList.add(new Student("Lewis", "Peterson", 90, 85));

        for (Student obj: studentList){
            System.out.println("Name: " + obj.getStudentForename() + " "+ obj.getStudentSurname());
        }
    }
}

import java.util.ArrayList;

public class Main {
    static ArrayList<Student> studentList = new ArrayList<>();

    public static void main(String []args) {
        addStudent("Adam", "Goldsmith", 70, 50);
        addStudent("John", "Smith", 20, 40);
        addStudent("Lewis", "Peterson", 90, 85);

        for (Student obj: studentList){
            System.out.println("Name: " + obj.studentForename + " "+ obj.studentSurname);
        }
    }

    public static void addStudent(String forename, String surname, int coursework, int test) {
        Student newStudent = new Student(forename, surname);
        newStudent.setForename(forename);
        newStudent.setSurname(surname);
        newStudent.averageMark(70, 65);

        studentList.add(newStudent);
    }

}

It was not due to undefined size but was because you were trying to access it without creating an object from a static method.
So just write static before it and it will work.

thought the above answer answers you question, but few words about static vs non static modifiyers in java

Characteristics of Static methods

• A static method is called using the class (className.methodName) as opposed to to an instance reference (new instanceOfClass = class; instanceOfClass.methodName.)
• Static methods can’t use non-static instance variables: a static method can’t refer to any instance variables of the class. The static method doesn’t know which instance’s variable value to use.
• Static methods can’t call non-static methods: non-static methods usually use instance variable state to affect their behaviour. Static methods can’t see instance variable state, so if you try to call a non-static method from a static method the compiler will complaint regardless if the non-static method uses an instance variable or not.

Non-Static methods

• A non-static method does not have the keyword static before the name of the method.

• A non-static method belongs to an object of the class and you have to create an instance of the class to access it.

• Non-static methods can access any static method and any static variable without creating an instance of the class.

So you'd better think if you need to define studentList as static or no, and if modify your code accordingly.

P.S. above written is taken from here

The difference between static (global, class level) and non-static (a instance of that class, an object) is important.

Creating an object by new Main() allows to work on that object and its fields.

The static void main(String[]) is the single entry point to the application.

Inside the main you have access only to static fields and static methods. So that is cumbersome.

package srivastava.arpit; // In a directory path srivastava/arpit/

import java.util.ArrayList;

public class Main {

    private ArrayList studentList = new ArrayList<>();

    public static void main(String []args) {
        Main main = new Main();
        main.execute();
    }

    private void execute() {
        addStudent("Adam", "Goldsmith", 70, 50);
        addStudent("John", "Smith", 20, 40);
        addStudent("Lewis", "Peterson", 90, 85);

        for (Student obj: studentList){
            System.out.println("Name: " + obj.studentForename + " "+ obj.studentSurname);
        }
    }

    public void addStudent(String forename, String surname, int coursework, int test) {
        Student newStudent = new Student(forename, surname);
        newStudent.setForename(forename);
        newStudent.setSurname(surname);
        newStudent.averageMark(70, 65);

        studentList.add(newStudent);
    }
}