From the photo i can't the features that would tell me it is a brassica, I would need to see the whole leaf and lobe shape at the bottom. Wild brassicas are very varible in size and shape to some extent, esspecially as it arible land, they seem to get cultivated genes in them. The ball mustard that grows llanelli morfa gets nearly as large as high as you discribe and has large leaves, but the leaves dont look like that.
Yours looks more like pickly lettuce or wild lettuce, these are obviously composits not crucifers though.