Measure, measure and measure again, definitely don't guess. Measure your space and decide from there what dimensions will fit best. There are a few rules that you can apply to choosing the correct size rug for your room:
Avoid 'the floating rug' look in which you have a small rug in the middle of the room touching no furniture. A good purpose for your rug as well as adding colour or pattern, is to create an area which brings all of your furniture together, meaning you should try to get a bigger size so that it is touching or goes under furniture.
If you are not sure what size will look best in your room you could try piecing together paper/newspaper and cover the floor space you imagine your rug will cover, this will give you an idea of size and if you will need to go bigger or smaller. Remember, the larger the rug, the more dominating the colour/pattern will be in the room.