There is only one thing that is important to concrete work, and that is how it will look when you are done. Everything else is simple, but making it look nice is an art, and something that takes time to learn how do do. The smaller you make it, the easier it is to do. The difference between an easy job and getting in over your head has a lot to do with the weather and how much concrete you can put down at one time and how many people you have to work it.
Size wise, it's real easy to do a nice job in small squares or rectangles. You have have smooth it out, and being able to get to it and work it before it hardens takes time to be able to do well. Doing smaller areas means that a beginner can do it fast enough to make it look decent. Trying to do a big area on your own for the first time will gurantee a disaster.
How old are you? What kind of shape are you in? How hot or cold will it be? How much concrete wil you be trying to work? How many people will be doing this? how particular are you on how it comes out?
Eddie