Ditto to what others have said .... there's lots of patchwork ♥ patterns online. Maybe you can find one or more that you could steal/borrow ideas from!!
As for size ... it really doesn't matter. It's more about how large do you want it to be?
Graph paper can be your friend ... draw out what you want it to look like. That'll give you the finished piece sizes/shapes. Then remember to add your 1/4" seam allowances on all sides.
If you don't have graph paper on hand, here's a site where you can print your own. It gives you lots of variables as to what type you can print ......
http://www.incompetech.com/graphpaper/square.html