It very much depends on what you do, and where.
For many, the 6m2 IS the main kite
F.ex Cape Town waves, where the 6m2 is the most used size in the summer.
For the light wind days maybe an 8m2, and of course you need smaller kites than 6m2 too.
I've been overpowered with my 5m2 often, down there.
Here in Denmark, a 6m2 is also sometimes too big for me (78kg), so I could never get away with such a size as my smallest kite.
4 and 5m2 can be used relatively often when good conditions, but you CAN get away with a 5m2 typically, so the compromise many chooses.
7m2 is by far the most used kitesize around here in waves, for most waveriders though.
So for me a 7m2 is a "medium" kitesize, and the main kite of course
You are talking about twintips, so you dont need a really small kite, because you avoid riding waves..
And at your location, it seems that a 6m2 is NOT the main kite - so must assume you have very low wind in your local area
So it is all about where you come from, and what style you perform (waves or tt), and what you seek and enjoy the most.
Some LOVE the days when it is windy, and live for those days, and have small kites just for that, even if not their most used kitesize.
That is another angle, and I think it goes for most kitesurferes actually
So instead of having a really big kite that just collects dust because you dont want to ride with that "dog" - you have a really small kite (or kites) instead and live for those days !
Talking about typical low pressure areas here, with all wind conditions possible, from 0 to 45 knots or more, without being "gusty" (like other low wind areas experience, as high winds only occur when "stormy").
By the way, JGTR is correct - a small kite has a much bigger windrange, and it has sufficient power without ever feeling overpowered.
One has to try to understand this I think.