Well, it doesn't really sound like there's a lot of 'garden' room in your backyard. So, one assumes that you don't have a wheelbarrow and lawnmower and other such items to store, right?
First things first...take stock of what you want this shed to store. What is it that you want this shed to do? Having a shed just for the sake of having a shed might not suit your needs without a little prior planning. Make a list. If you're lookng at just a few pots and a very few small garden tools, then you might be able to deal with a shed as small as 3'x6', or 4'x 6'. Neither would look too obtrusive and could be placed in a corner of the yard. Is the yard fenced?
If your list includes garden furniture, then take the time to stack your furniture in a fashion that you might if you had a small shed to store it in. Then measure how much room it takes up. Assume that you might be able to 'hang' some items from the underside of the roof.
Once you've made your list and taken stock of your space needs, then do a little travelling around your area to retail places that sell sheds. Pick a style that pleases you, get a lists of available sizes, and then go home. Measure out the size you think you might want on the ground right where you might place it, think about it, and then make your decision.
Small sheds for a yard your size can be purchased quite reasonably, unless you're into carpentry and want to try building yourself.
Oh, almost forgot...you want potting space, too, right? If you think in terms of a rectangular shaped shed with a door in the middle of the long side, then put your storage needs to one side, and potting to the other. Many shed manufacturers outfit their small sheds with potting tables these days for a few dollars more.
[This message has been edited by treebeard (edited August 25, 2003).]