If you're doing a postcard, it probably doesn't matter. 

From what I've read the typical response rate is 1% to 10% depending on what type of piece you're mailing (post card, zip letter, typed letter, handwritten...)

@Michael Quarles has a great site where you can choose which one you like best---and he has some super choices. if you go to his profile here on BP you will see the link.He also sells lists ;-) I love OOS (out of state) owners and free&clear lists.Some people use the same postcard every time; others will use the same one for 4 consecutive mailings---over a period of 4-6 months and then switch.

I have tried oversized and undersized postcards in split tests, and they just dont convert as well. 

If you want to check out samples of postcards I have sent out I uploaded a couple in the BP fileplace.Frank

If my first mail piece was a yellow letter and now I'm sending a postcard, do I change all the numbers-- and now need 18 numbers-- or do I keep one number throughout the whole campaign?

Late last year I sent out about 6,000 postcards and got exactly 4 calls.  3 to take them off the list and 1 from someone that wanted about 60% more than I could pay for the property (And they didn't even call about the one I sent the mail about it was another place).Did I stop the mailing?  

don't send more than 1000 yellowletters first month then 1000 of zip letters followed by 1000 large text postcards.  

.% Equity is really the most important tho because you don't want to waste the 50 cents per postcard x 6 to 12 touches per year on someone who has 10% equity in their house (for the traditional cash offer), when you could have spent 10 cents one time to make sure your list only has people that can afford to sell their house at a 25-30% discount (without bringing money to closing).Does that make sense?

This morning I ordered a 1000 letters from yellow letters.com as well as a list of namesMy criteria for the list came from advice found on the forums from Michael QuarlesAbsentee 7 year ownership min 40% equity SFR 2-5 BedroomsOwner occupied 15 year ownership owner=65 or older min 40% equity sfrI have divided my list into four sections and will send out 250 yellow letters a week for the first monthI will do the same with a follow up letter the next month, and a postcard the next month to complete a 3 month campaign.I am excited that I have FINALLY taken action