Hi Isabelle,
after all - there are no rules at all.
It depends upon.... what YOU are trying to achieve.
In general - flats lack the third dimension, the depth. You try to bring depth by painting them.
So, there is an approach to a (small) diorama in placing the figures narrow (regarding depht). Regarding left / right distance the scene you try to express, determines the distances.
If you use 3dimensional parts (houses, trees, vehicles) this attempt "as narrow as possible" is nearly at its end.
My advice - cut a piece of cardboard 3 cm "deep" put it on the table and arrange your figures.
If it works for you - fine.
If it does not work - try 5 cm deep.
For this Russian scenery 3cm was suitable
![](http://i1043.photobucket.com/albums/b433/Joerg12/flat%20tin%20figures/Russ-1812.jpg)
here too
![](http://i1043.photobucket.com/albums/b433/Joerg12/flat%20tin%20figures/zerbst3a.jpg)
for this the number of figures was to much. Depth is 5 or 6 cm
![](http://i1043.photobucket.com/albums/b433/Joerg12/flat%20tin%20figures/SANY4247-1.jpg)
Here the depth is 20 cm. Not really suitable for mounting at a wall, but there was no other way (for me) to create the depth of landscape.
![](http://i1043.photobucket.com/albums/b433/Joerg12/flat%20tin%20figures/1-SANY6508-100_zpsf12187ea.jpg)
Hope this helps a bit.
Joerg