hector mapping map size

Hello everybody! I am currently mapping a small tunnel, with about 200m in length and I'm using hector slam with a laser scanner. All working good so far, but with the exception that the map size is not big enough. I have to set the maps size manually via map_size in the launch file before I really know how big the map will be. This may lead to moving outside the map area and not being able to map any further. Setting up a map_size of 20000 etc. is not an option, since it would use up too much space. Is there a way to dynamically increase the map size? Cheers and thanks in advance Bobby