Scaling from a walrus would suggest 500-700 lb, but I'm not sure if that's the body shape or not.
The first line of flavor needs to be "used as pack animals..."
Anyone wanna buy a used pack animal?
Updated.
Since they are used as pack animals, we ought to include carrying capacity (and possibly a training section).
I had 229.5 lb as the upper end of Medium; don't you always round down in D&D?Yes, since it says they're friendly and easily domesticated we should have their training DCs as regular Animals, without the usual +5 for Magical Beasts.
As for carrying capacity, they work out up to 114 lbs for a light load, 115 to 230 lbs for Medium, and 231 to 345 lbs for a heavy load.
I had 229.5 lb as the upper end of Medium; don't you always round down in D&D?
I'm ok with stating they can be trained as animals. I guess we should come up with a price. They should be worth less than a hippogriff, I think, since they can't fly. Young are worth 1000 gp and training costs 750 gp?
500 gp training was another option I had, so that's fine too.
Ahh, I thought I'd gotten 230 for 1.5*154, but I think I got 229.5 for the upper end of medium load, rounded down, and took 230 for heavy. Might as well use your numbers and just round up on the medium load.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.