There is no maximum map size, however the power and resources available to your PC will dictate how big your map can be before you notice a performance reduction.
If you find your PC is struggling with a large map, it is possible to break your map into a number of smaller individual maps, then have one main map consisting of branches containing links to all the other maps.
To do this, select the branch you want to break off into a separate map and select "Save as Linked Map".
This will create a new map consisting of the selected branch and all it's child branches. This separate map can be accessed by the link in your main map.
Save as Linked MapDouble-Click Link to Open