Just ordered some 5.5 x 8.5 and waiting for them to be delivered to me and then I will deliver to my clients with self addressed campaign back to me. I want to use them in a reverse fashion and see how many people respond to a call to action request.

5 X 7

I'm not sure what you mean by "unaddressed." But normally I just use a standard sized postcard.

we run 4.25x11 which works for EDDM.

5x7 pre-addressed.

I do no post card mailing that require me to print and apply a label. That takes the cost WAAAAAAaaaaaaayyyyyyy UP!

4" x 5.5"

I use 5.5 X 8.5, and use EDDM for distribution

we like the jumbos 5.5x8.5

I use 5x7. 

Half sheet 5.5x8.5 for those that wear glasses lol. 

5x7

I use 5 x 7 for every day but every now and then like the bigger ones to make a splash.

More bang for the buck on the largest size.

The last few times I used 5.5 x 8.5. They seem to be becoming more popular. But I have used 4x6 and 5x7 also.

When I use postcards they are normally 5X7.

I go with the largest size available.  I am just trying a bi-fold

Standard post card size, 4.25x5.5?

The smaller ones so I can use postcard stamps.

I like to be consistent at 8.5 inches

4 x 5.5

It depends how many I am sending.  If it is under 200 I use 200 jumbo.  Anything more, I use regular postcard size.

4 x 6

Nothing less than 6x9

What are postcards ????

I no longer use post cards but prefer the larger sizes that are commonly used in our market.

parked and reading

Jumbo! I like having 3-5 photos and a call to action. Very little text otherwise.

I evaluate the ROI as part of deciding upon the size to use for any mailings, if a mailing is appropriate.

Standard size.

The larger the more visible and effective. The small ones are completely useless. Colored ones are the ones people glance at .....

The people's attention span is about 1 sec before trashing these cards.