Here is a comparison to give you an idea of sizing. Here are some real world examples.
     1.5 x 3.5: About the size of a Band-Aid
     2 x 2: About the size of a Saltine Cracker
     3 x 3: About the size of a Post-it Note
     3 x 5: About the size of an Index Card
     4 x 6: About the size of a Mobile Phone
     5 x 5.5: About the size of  a Square Paper Napkin