Does fin size matter? Yes - but 'big'/ 'long' - isn't necessarily better.
Long gives you more propulsion. You might want 'long' when you encounter a strong current.
But, 'long' means you are more likely to hit that precious coral. And, 'long', if you aren't fit, means tired
legs quicker than from smaller fins.
Snorkeling is usually neither a marathon, nor a sprint. It's a meander, except in a strong current.
The flipper can get in the way!
I started with 'long' but now prefer 'shorter' for both manouverability and lack of leg cramp.
Fins have to be comfortable. Blistered toes after a few hours snorkelling on the first day is no fun.
Obviously, it's the foot moving in the fin that leads to all the blisters, including the toes.
Avoid old-fashioned, fairly inflexible 'saddle' fins. Any foot slippage in an inflexible 'saddle'
leads to blisters in very short time.
A backstrap can allow foot slippage which leads to rubbing (tighten the strap to minimise obviously).
A reasonably tight-fitting fin with a tight but elastic 'saddle' or strap over the foot, with a back to the
fin seems to minimise rubbing and blistering.
Split fins give more speed but less manouverability than solid fins. Manouverability is needed to follow
fish and swim around coral.
Rarely is there a need for speed.
Vital consideration. Are you going to wear 'reef' shoes? Negatives are, another item to
remember plus putting on and pulling off.
Pluses:- comfortable, reduce blisters and lessen the discomfort of walking ashore over coral rubble.
Reef shoes affect the choice of fin size. So make that decision before you buy your fins.