If the room is restricted by size it is often best to order the largest size possible and re-arrange the chair configuration or allow the carvers to hand off the edge of the rug
In an ideal world it is easier to maneuver chairs if they sit on the rug when pulled out. It is always a compromise to have the back legs half on half off. The minimum that will allow for this is 75cm clear of the table in all directions.
Remember the centre of the rug will be obscured by the table and chairs so focus on a rug with a border detail. Secondly, think about the type of rug you want, flat weave and low pile rugs are better under chairs and less susceptible to getting dirty.
As no two tables are alike often a custom solution may be the best answer, a rug tailored to the space. I have a couple of simple tips that may help here.