I am in the process of finishing part of our attic to be a 4th bedroom. I have found that the size of roof rafters vary (2x6, 2x6.5 and 2x7). How do I even these up so I can install a plasterboard ceiling?

You can install furring strips perpendicular to the rafters every 16" and shim out the furring strips until they are straight.

Use a install a string line from one end of the attic to the other end at each row of furring strips and shim out the strips to the string line.