Your thread seems to have been missed so I'm hoping if I reply it will bump it back up a bit.
I think the best depth you'll need will be 2 to 2.5 ft and then the size of the pond is up to you ,if you only have 3-4ft then lots of others on here have managed to make lovely ponds around that size.Use the search function,top right corner and you should find loads of great photos and info.
I,m sure if you go ahead you'll be amazed how quickly different things will move in.
This is my little effort,it's about 6x6ft ,was put in around April and I have already got 1newt 1frog ,pondskaters,watersnails,water beetles and loads of larvae.
You can make it to please yourself really,but it's important that one area at least is shallow enough to let small animals scramble out.
Go on ,do it,you'll enjoy it.