Charlie I find that sowing Brussels before say 1st May gives too-early maturity, for most people who want them as food from about mid Octber or later. Eve n supposedly late varieties button earlier from sowing in April.
However if you want Brussels in September or you are in zone 7 or lower, yes you can show them earlier.
Doass, planting size is not fixed, depends on weather etc, I plant cage ages at any stage after they have about two true leaves, but that is a guide only. Planting small is generally worthwhile for rapid establishment.
