The square bale is the one you stack, and that is exactly why children get so much out of it. Kids Globe puts four of them in a bag, pressed from flax straw, with the coarse texture and straw colour of the real thing. The card calls them 'Square Bales'.
They are made to 1:32 scale, which puts them with the usual toy tractors, Siku included. On the trailer they go down in layers, crosswise over one another the way it is done, and in the barn you can build a stack right up to the ridge. A child who would rather feed than stack lines them up along the feed rail.
Each bale measures roughly 7 by 3.5 by 2.5 cm. That is easy for small fingers to handle, and small enough to lift one out with a front loader without toppling the rest. Four bales per pack.
One of those add-ons you buy once the farm is already standing - and that then turns out to be the most-used part of the whole barn.