I also use Reunion.
It is possible to export a subset of your tree from Reunion to a GEDCOM.
What you might consider doing is go to your own profile in Reunion.
Then look for the MARK option in the lefthand column. A menu will pop up. Keep Action as Mark; keep generations as 10 (for now); Under "Who" select Ancestors from the menu; your name should then appear in the next box next to "of". Select Unmark everyone first, Mark Source Person and Mark Spouses. Leave "Mark parents of spouses" and "Mark siblings of ancestors" UNmarked.
Then click on MARK.
THEN, go back to Family View.
From the FILE menu, select EXPORT, GEDCOM. Under who, select "Marked people and their spouses". Include Sources, but don't bother with multimedia links. Then click on EXPORT and follow the rest of the instructions for saving a GEDCOM file. Note where on your computer it is saved.
This should give you a GEDCOM of less than 5000 people.