If you have the room, make it as large as you can afford. If you want crystal clear water you will need a decent pump, filter and UV filter. If possible (unless it will be covered) put it in a location that does not get full sun all day as you might be cursed with 'blanket weed'. This is the pond keepers biggest headache for maintenance. Also, be prepared for cats to be looking in and fishing if you do not have a net. I live on the south coast and lost a few fish to a seagull until I put a net over the pond.
We had a fair sized pond over the back of the Orchard when we first moved in. I did have a go at Maintaining it, but it was a bit of a bind TBH. All the fish kept getting eaten my a Heron and the edges were a slug haven, not to mention the midges and other nasties that hatched from it.