the best size to use is the 4" x 6"... regular postcard size.... people will throw it in a drawer and keep it for future use.... the jumbo is more expensive and they DO NOT SAVE THEM.....

I haven't read the other answers yet but I'll go with whatever Barbara Todaro said!

The smaller is more personal and held easier by the hand. Less is more

I like the larger ones myself, and use them in my farming

I get more calls from the larger ones - and when I show up for a listing appointment, they've often saved them too!  The postage cost is minimal for the benefit here.

I personally use the 6x4 postcards to market.

Either, most important is the attention grabber on either side otherwise it goes in the recycle bin without much of a glance

Bigger is better usually, unless the postage is a lot more.

I say the larger will get more attention. I read and then throw away storing the info in my head. I don't save any of them. Can't stand clutter.

I trust Barbara Todaro , she has been doing this forever and should know. 

We use long and narrow; whichever fits your budget

People keep the 6x4 cards but not the large cards. I only use 6x4 and have received calls from mailings I did 2-3 years ago.

I see more larger ones being mailed out than the smaller ones.

What ever size so it doe not look cluttered

Regardless of size ask yourself what benefit you are giving on your card.

The larger size. 

Wayne and Jean Marie Zuhl I believe the larger one has more impact.

Wayne,

It depends on what is on the card, how it looks and what it says.  A

I think the call to action if more important then the size. IMHO

1/2 sheet I do 5.5x11.

Bigger is better, in my book.  Stands out and doesn't get lost in the shuffle of other postcards.

Whichever accomodates your message adequately... or whatever Barbara Todaro said.  LOL.

My preference would be the larger one. My thinking is that it would stand out more than the smaller one.

My sense is that this may be a market specifc answer.  When I mailed ... I used the larger cards.

Obey Barbara Todaro.  She knows what she is doing.

I use the 6x 4 and get more post cards out for a better price

Barbara Todaro nailed this one!

My first thought was to ask Barbara Todaro and she has already responded!

6x4

The size is likely not as important as is the consistency of delivery.   

I use the Jumbo size.

Wayne and Jean Marie Zuhl - unusual size matters....

Much prefer the smaller size.