In the story we read:
The next night Rabscuttle, who had been told by El-ahrairah what he had to do, went to the garden and dug a scrape. He hid in the scrape all night; and the next morning, when the children were brought to play, he slipped out and joined them. There were so many children that each one of the mothers and nursemaids thought that he must belong to somebody else, but as he was about the same size as the children and not much different to look at, he was able to make friends with some of them.
Since Rabscuttle was Captain of Owsla for El-ahrairah, he must be a full sized rabbit. So the children of King Darzin and his followers are similar in size and appearance to full grown rabbits.
Some fandom sites suggest that this means the King must be a Hare. However, we also read that:
“Now, as soon as Rabscuttle got inside the King’s palace, he scurried off and went into one of the dark burrows; and here he hid all day.
Hares don’t burrow, so this probably rules that out as an option.
In The Story of El-ahrairah and the Black Rabbit of Inlé we learn that the King’ soldiers:
couldn’t get down the rabbit holes. Some tried, to be sure, but they soon came out again, because they met El-ahrairah and the other rabbits. They were not used to fighting in narrow places in the dark and they got bitten and scratched until they were glad to come out tail-first.
This confirms that they are bigger than rabbits, and have tails, but that they are not used to rabbit style ’in-burrow’ fighting, despite having burrows of their own.
El-ahrairah believes that King Darzin and his troops will be susceptible to the diseases that afflict rabbits as he tries to infect himself with ‘the white blindness’ (I assume myxomatosis) from the holes where
lie all the plagues and diseases that come to rabbits
In the end however, when the Black Rabbit and his shadow owsla come down upon King Darzin and his troops
They turned and fled. They vanished in the night; and that is why no rabbit who tells the tales of El-ahrairah can say what kind of creatures they were or what they looked like. Not one of them has ever been seen, from that day to this.
If the tellers of the tales don’t know themselves what manner of animals they were, then we mere readers of the tales can hardly be more sure.
The best we can say is that they are bigger than rabbits, like them in some ways but unlike them in others. (Unless there is other information in the Tales from Watership Down book, which I’ve never read.)