The dimensions would be fine and deep enough to over winter fish. I suggest if you are only putting sand on the bottom that you line the hole with a fleece also, as stones in the side may also puncture the liner.
When choosing pond plants the size of the pond matters so check this out as 1 large water lilly would be enough with a couple of marginals such as Iris?

I am confused as to why you would want a pond on your decking... Ponds are damp, wet areas... decking, of necessity, dry.

Ponds and decking is quite popular MG as are hot tubs my near neighbour has a swimming pool surrounded by decking.

Oh well, I prefer my pond to look natural...

I do have a natural pond (13 years now), I rely on nature and not a pump etc MG, (I thought of installing a pump last year and decided I like what I already had). but surely you can have this in a modern setting too?
It seems to me that we have to find a way to mix the natural and modern ways or lose out to so called progress which is unsafe to the planet. Man lives 3 score years and 10 and the damage he does in that time is way beyond that span.
It is possible to put a natural pond in decking but for most green water for a few weeks is not acceptable so in goes the pump and chemicals, sad but that's the way it is! But occasionally someone thinks about the bigger picture and waits for nature to work!

Some progress is good but not all. And it looks as if we will all be living longer than 70 which is causing other problems.

I'd say 'I'll stick to my natural garden' but with the amount of rock we brought into it it isn't natural either I guess

But you are not doing damage and that what counts MG. I dont want to live to be old if it means being in pain. infirm and in a nursing home!

No we are not doing any damage...