Making round buildings can be suprisingly hard in Minecraft. Luckily there are some tools to help you on this issue.
DonatStudios.com has a handy tool for generating circle floor plans for your buildings. You can choose the width and height of the base of your building and it will automatically generate a plan for you. There are also some other settings aswell, be sure to check them out!
By clicking on the PNG button you can download the instructions as an image, so you can open it and view it at the same time as you are constructing your building.