Small standard donkey would likely be below height of the average Fjord. Large standard will be equal or taller. Now, if you're looking to create a mule, remember back width and head size can be affected by the donkey size, so depending on what you use, special gear may be necessary. (Think mammoth jacks used for draft mule production).
By the by, you may know this already, but Fjords are already more donkey-like than the typical horse. They stop and think instead of react...they do seem to be more helpful and considerate than donkeys. Donkeys are thinkers, but rarely for anyone's sake but their own. Also, Fjords are easy keepers on grass and don't have the health issues most horses seem to have. It is hard to find a taller specimen, though, which is unfortunate for the bean pole population.
Are you thinking of creating a mule, or looking to replace your Fjord with a donkey? So curious...
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