Peter Berry of Baccus 6mm fame shows his figures with 60mm frontage.
This serves mutiple purposes, the most importance of which is he gets
to sell more figures that way, but also this matches the 25mm scale
DBA elements, and he has decided to go with 60mm frontage for all of
his Polemos rule sets, the Ancient version of which comes out late
this year or early next year.

Now I have seen DBA elements with 6mm figures done in both 15mm (40mm
frontage) and 25mm (60mm). the visual results for 60mm frontage
bases are truly worth more than the +50% increase in base sizes.
Unless you chose 6mm figures because you are a poor student (like I
was 15 years ago) or have no space in your house, the main purpose of
having 6mm figures is to make your army look like an army. There's
really no comparison in that "army" feel between 4 25mm figures on a
base and 72 6mm figures on the same sized base. One looks like an
army, the other doesn't. You can also do wonderful dioramas or
create wedge, diamond, and Cantabrian circle formations with your
mounted elements, show elephants and their attached skirmishers, show
epilektoi running out of hoplite formations, or Roman legionares in a
checkerboard formation with the 60mm frontage bases that you just
can't do as well with 40mm bases.

All that said, here are the reasons I continue to use 40mm frontage
bases with my 6mm figures even though I now have enough money and
space to go larger if I wanted. First, I have so many armies in 40mm
that it would take a ton of rebasing to fix everything, and many of
my paint jobs are "themed" in certain colors that I probably can't
easily replicate. Second, like to play BBDBA as much as I like to
play DBA, and I don't want to make 36 elements at 60mm frontage.
Third, I can choose to play DBA with more figures simply by making
every four regular 40mm elements represent one giant DBA element
(with 80mm frontage placed 2x2). I just rescale (ie double) all the
board, movement, and ranges. A hoplite phalanx of 8x 4Sp elements
uses 32 of my bases of hoplites and would have a total of 512
individual hoplite figures--pretty cool.

6mm figures are best used to show masses of troops and present
formation and action dioramas, so if you want to do this I suggest
60mm frontage elements (either use DBA 25mm scale base sizes, or a
standardized 60x30mm if you play Warmaster/FOG or other rule sets as
well). Otherwise stick with 40mm frontage elements--you can more
easily play DBA games with all the 15mm army owners out there, you
get smaller board size, you can more easily play DDBDA, and you can
always go 4x if you want to put out more figures and still play
straight DBA.