Making a "toise" was one of my projects this winter.
Essentially, a "toise" is a height gage used to measure your sheep. Size, or more accurately "height measured at the shoulders" is one of the important criteria (but NOT the only one!) for selection in the official standard of ouessant sheep.
When measured at 3 years of age, a ram must not exceed 49 cm (that's 19.29") and a ewe must not exceed 46 cm (18.11"). That said, most breeders prefer that their sheep are at least 2-4 cm under this top limit!
So it's not surprising that most serious breeders measure their sheep and are careful to select and breed from animals that are well within the standard.
The GEMO (Groupement des Eleveurs de Moutons d'Ouessant) site has plans that you can use to make your own "toise".
Here's what my finished "toise" looks like :