ExSync is a 32bit application. A 32bit application can use up to 2GB of RAM on top of the operating system up to a total of of 4GB.
From ExSync 3.0.0.1 up to 4GB of memory can be used on Windows 64bit (but not on Windows 32 bit) This effectively doubles the amount of memory available. As a result ExSync can cope with much larger datasets.
Since every database is different it is important to get to know your data and monitor the amount of memory in use in the task manager.
Watching the process will enable you to know how much of your database ExSync can cope with at once.