The boil is important for achieving certain beneficial changes in the chemistry of the wort that include the dropping out of haze creating proteins. So don’t forego the boil, even if it’s only a 6-liter partial boil.
The main issue with boiling a small quantity of wort is that you'll get caramelization a lot sooner than if you were to boil the extract in a full 25 liter batch. Caramelization may make the beer a lot darker than its style requires if you are making anything lighter than a bock, and it will also give the beer a kind of burnt flavour. If you are making a dark beer anyway, then it doesn’t matter that much, but anything lighter than a bock will be noticeably darker and taste a bit scorched. But it will still be drinkable; just not “consistent” with the style you might be shooting for.
As such, I would boil 6 liters of wort for no longer than 45 minutes. A full batch, in comparison, would be boiled for approximately 90 minutes.
Remember also that you will get evaporation and if you were to boil 6 liters for 90 minutes, you would lose over 1/2 of your wort! This is why you would actually start a full batch with around 30 liters of wort. After 90 minutes, this would give you 25 liters remaining, and all that extra water will have prevented significant caramelization from taking place.
Also, make sure that the water that you are going to pour the 6-liter batch of boiled wort into is pristine, as boiling also acts to sanitize the wort and drive off any chlorine (such as from tap water). I would recommend getting a 15-liter jug of “spring water” to mix with your small quantity of wort. These are the jugs meant for office drinking fountains and will contain no chlorine.
In summary, definitely do a boil, even if only a 6-liter partial one, but for no longer than 45 minutes, and use a mixing water that is chlorine-free.