well i'd recommend using delegates to pass values from one form to another form, as long as they are short, but if you need pass a lot more information, you can reference the form1 (the owner of the comunication) and then use the referenced object on the form2.
you can do it this way.
//lets supose you have an already defined form2 class
//here you pass the form1 or mainform by reference
Form2 form2=new Form2(this);
//and in your form2 you will have something like this
//Constructor of the form2
public Form2(Form1 form1)
//and now you can obtain and even modify the form1 from the form2
form1byreference.Size=whatever you want;