Greeting Cards: Getting Started
This will be a quick lesson in desktop publishing believe it or not. 
There is more to a greeting card than just the artwork. The end result is printed artwork on a piece of folded paper inserted into an envelope.
So given these three items (envelope, paper, artwork) we will work backwards starting with the envelope and ending with the artwork.
Envelope
The greeting card should eventually be able to fit in a standard sized envelope. Envelopes come in standard sizes. For the purpose of this challenge we will use Announcement Envelopes.
Announcement – These are very common envelopes. The flaps are usually pointed but can be flat. They are mostly used for announcements, invitations, cards, small booklets, brochures or promotional pieces.
Paper
In order for the card to fit properly it must be smaller than the envelope once it is folded. Therefore you have an envelope size and an enclosure size.
Envelope ................... Enclosure
A-2
4.325 x 5.75 .............. 4.1875 x 5.5
A-6
4.75 x 6.5 ................. 4.625 x 6.1875
A-7
5.25 x 7.25 ............... 5 x 6.875
A-8
5.5 x 8.125 ............... 5.25 x 7.75
A-Long (Slimline)
3.875 x 8.875 ............ 3.75 x 8.625
A-10
6 x 9.5 ..................... 5.75 x 9.125
For the purpose of this challenge you may use any of these enclosure sizes as long as the card is flat and not folded. For a folded card the entire piece (including bleeds and crops) must fit on an 8.5 x 11 sheet of card stock. I’ll be making some templates and working with each of you to build a card file with you art work that you may print. This file will be a high resolution PDF complete with fold, bleed and crop marks.
Artwork
The end result must be high resolution/print quality. This means a great big file. Files should be 300 dpi (dots per inch). If you don’t know or understand the difference between screen resolution vs. print resolution, don’t feel bad that’s common. BUT I’m not going to go into it here so ask if you don’t know and I’ll be glad to explain.
Your artwork can be portrait or landscape.
For those using Photoshop just build a file the size of your piece (be sure to include a small bleed IF your image will go off the page) at 300 dpi.
I’ll talk to the girls about how to get a specific file size in poser/DAZ.
Like Lessa said it might be better at full size. If not I can repost.
Now... to your questions. Really all you have to do is make the artwork. But to get the size right you need to think about the end result which is the card. Do you want your card (s) to be horizontal (landscape) or verical (portrait)? Do you want the artwork to bleed or will it have a white border (easiest option). What size envelope do you want to use because this effects the card.
I'll be here to help you with all those questions and to make a file for your card so you can print, trim and fold it properly. So really all you need to do is make some pretty artwork to go on it
But it's alot of work and say you wanted to do Holiday or Happy winter cards and print them for several folks. Making a special envelope for each one that will make it through the postal system would be a ton of work.