I start my brassicas off in pots - 2 1/2", then on into 4" - but I do it to avoid club root which is in our soil. By growing them so large before planting them out in the soil, it gives the plants the chance to grow a good root system before they get attacked. You really have to watch out for the plants getting root bound because they don't like that. The mix I grow them in is multipurpose compost mixed with seaweed meal, wood ash, fine manure of some sort, and this year worm casts.
If you have no club root in your soil and you are in an equable climate, (and you can keep the flea beetles off
), then you are better to sow brassicas in a well cultivated seed bed, then plant them out into their final positions when they have about 4 true leaves. Apparently the slight trimming of roots caused by moving them gives them a growth boost.
The brassicas I grow are PSB - purple sprouting broccoli, summer and winter - caulis sometimes, sprouts sometimes, dwarf green curled kale always, Pentland Brig kale (because it's Scottish and nearly fell off the official lists a few years back), also Brukale, which has various names like Petit Posy, but is basically like a blown Brussels sprout, but crops away like mad. I grow savoy cabbages, but I find all cabbages here are badly affected by slugs, and it's too cold to use a bio remedy, and if they survive the hens get them. I don't grow any crops for the sheep, but they get any overblown brassicas, which they love. Our sheep don't eat carrots!