Enlarged Marine Animals need a reason to be large
I believe large seahorses are more than possible. Anatomically, seahorses are simply fish, they have no relation to horses let alone any mammal or land animal.
As they are simply fish, we have seen enlarged fish and sharks to enormous scales - the largest currently being the Whale Shark:
You seahorse does not need to be this big though, so perhaps just a sunfish size is adequate as follows:
So given we have examples, we know it would be possible.
Now for evolutionary pressures: there is likely a reason why Seahorses haven't grown to this size yet. That's because larger animals need much more food to sustain themselves, and much more vulnerable to ecological and environmental change. Larger animals tend to have to travel further, to abundant 'small filter food' areas that have a seasonal variation, so the size becomes an advantage as you can support longer endurance-style swims in comparison to smaller fish.
So your seahorse will need to eat lots, swim far, and likely be a filter feeder.
Unfortunately, sea horses are not efficient swimmers - in fact they are the slowest fish in the world. This is because they are vertical, and use their dorsal fins for propelling, in lieu of their tails. That makes sense though and is ok, as the appeal of being a sea horse is being sexy amongst the safe coral, not in long endurance swimming.
So if it does happen, to be large your sea horse may actually start looking like a normal fish instead.