First off, see what the ideal dimensions and DPI are for each product. You take the inches (say 5) and multiply it by the DPI (say 300) so you know for that direction, you need 1500 pixels. It is a pain to make this stuff in paint because it doesn't let you zoom out past 100%. If you have some other program that can do that, great! Draw your designs and pay close attention to bleed area and such, keeping important things within the danger zones.
I'd say that most men's T-shirts have pocket size designs on the front, and full size ones on the back. If you make a bumper sticker design, you can fit it on all the mugs and even other places like two of the hats (but not the black hat). The Boxers are worthless because you can't design anything on the back. If you're going to make detailed posters, you should probably own Photoshop or even Photoshop Elements. You will need it to manage such a large picture, I'd think. You should know that stretching out a pic will leave it pixelated unless it's a vector pic, such as an Illustrator file, but illustrator is hard to get used to and it costs more than Photoshop... So, if you make a poster, you need to make sure you either design it on that scale to begin with, or get very very very high resolution pics... Of course, even the shirts need high resolution pics if you aren't going to draw them yourself in a paint program...
YOu can make as many stores as you want on one free account. You need to devote each store to one concept. I think you can link to another store within that store, and even two stores, so if you make a lot of stores, you need to map out your connections... That's something I haven't done yet, but you may want to try it anyhow. Make sure to keep all the images centered, and don't use anything that remotely look like corporate logos or celebrities (with the exception of political leaders, because they are fair game) and try to come up with something clever that people would be willing to buy a novelty shirt or something for.
You can visit my stores here:
http://www.cafeshops.com/willworkformar
http://www.cafeshops.com/bobtheduck2004
EDIT: Oops, forgot to tell you what it was like. You won't get many sales off the site because they don't advertise for you. You need to do all the advertising yourself. The mens' shirts are great, but you can't wash them in hot water or dry them in much heat. That's the only product I own so far.
There are ways of getting your store on search engines, but I don't really know how. I can't find the submission page on Google... Word of mouth is probably the best, and considering the high price of buying online, a better thing to try is to buy 15 of the product in question and sell them locally. If you do that, the price goes down drastically. Even better is to sign 15 people up and have them pay you when you have the signiatures, and then buy them all so you'll allready have your money. If you tell it to buy from your own store, you get a 1/3 discount and all of your shipping is dropped from 4 dollars an item to 1 dollar an item. It's a very good deal, especially if you want to make shirts and gear for a group thing...
You have to be carefull about things like centering and cropping on the pictures yourself. It's all up to you.