Slitherine

if a unit of 2 base is hit by a unit 3 bases wide and has a 4 base depth. 2 bases line up for the impact phase(you can not expand right) but in the melee phase you get 2 v 6 bases ie you get the over lap. is this the extent of the bases you can get involved

Unless the 2 base BG is charged in flank or rear that is the max you can get in.

So one unit is 2 bases wide, the other is 3 bases wide,, 4 deep (12 bases in total).
Then the impact is 2 bases v 2 (4 dice each side)
In the melee phase the 3 wide unit would count overlap on one side, so get 6 bases; but it could expand by one file on the other side, too, and thereby get 8 bases in the melee.

>but it could expand by one file on the other side, too,

...depending on whose turn it is!