0.31 (12lb) or 0.35 (15lb) are the two i know about. Anything thicker causes resistance.
It's been reported the portoguese shore fishermen use 5lb mainline with a heavy leader & able to cast around 200 yards with a long type rod.
and Peter Bagnall (about 30 years ago) once did a cast with 4lb mainline on an Abu484 (they were new then - the latest thing) which achieved a considerable distance.
It's partly a matter of how controllable (backlash free) you want your casts to be versus 'ultimate' distance.
If you have a 6500 (or a 5500) overfilled with really thin line (say 0.20 mm) then, unless you go in for a lot of braking , you are going to get fluffing and lifting and, if you aren't a very smoooth caster, birdsnests.
If you drop the line level on the reel to reduce that you are applying another form of "braking" - so there is a compromise between thin line, full spool and minimal mag or oil braking and thicker line, not-so-full spool and lots of braking
with a really smooth (but powerful) casting action letting you go towards the more 'dangerous' end of the compromise.
In practice, almost no-one uses line of less than 0.25mm (and 0.28mm is more common) and seldom more than 0.35mm (and only that because it is the standard for 150 g + events in UKSF)
for multiplier casting in casting clubs or tournaments with weights between 100g and 200g ....
.... but I'd be interested to hear if someone regularly uses thinner for (say) demonstrations.
when you get under 0.25mm the edge for ultimate distance is definitely going to the fixed-spool