While 4 x 6 seems to be a smaller Postcard, they are great for a 'shorter' message.  Their cost is much less expensive than the larger Postcard size

The 5 x 7 Postcard is medium sized postcard.  It is about 50% bigger than its younger brother (4 x 6), giving it a lot more room for details without feeling squeezed.  The 5 x 7 is recommended when you have more details to print.

5 x 7 (med)

The 5.5 x 8.5 Postcard is classified as a large sized postcard.  It is almost twice as big as a 4 x 6 postcard giving it a lot of room for your details. It is so roomy that our example shown here is shown with
2 sections, giving it a lot of

rooms for details. 

The 5.5 x 8.5 large postcard

is also quite often used as a vertical postcard.  See this example.  The vertical card
gives a very different look.

Another popular size
for Corporate Class
Postcard is a 6 x 9
extra large Postcard. 
It looks similar to the
5.5 x 8.5 large postcard,
but the giant size makes it really stand out.

5½  x 8½  (large)

6 x 9 (x large)

The 4 x 9 Postcard is very effective because of its 'extra wide' image.

The example show that there is enough space to put in a main picture (focus) and there's enough room on the right to put in extra information, or for further description or fine print details

4 x 9 (large)

When we look at the 4 x 11 Postcard, we think of 'Wide Angle Lens'.  Look at the example, you can see the extra wide image.  The 4 x 11 extra wide postcard, along with another large brother, the 5.5 x 11 are mostly used by Graphics Designer for Corporate

Postcard

distribution.

4¼ x 11 (x wide)

5½  x 11 (giant)