The best way to make an initial decision about enough available space in the attic for the use you plan, before you do too much planning, is to take a tape measure to your attic and start making some measurements.

Try to take a look at the slope of the roof. Then consider the ceiling coming to about one foot less (frame, insulation and ceiling material). Then measure straight down from where you think you can have the walls and ceiling meet. Find a way to mark the flooring or the joists (chalk is a good idea) where the walls will join the floor. Then measure the area inside those marks.

This will give you a rough idea of the amount of space you have to work with. Next think about what furniture you want to put in the finished room.

If you plan to include a bathroom, allow for that, as well. Then go back and measure the furniture and other items you will need in the room. And allow yourself a 10% margin for error.

This should allow you to make an initial estimate.