Typically when people put in ponds, they wish they had gone bigger, so go as large as your space and budget will allow. We started with a little 90 gallon Walmart pond, then went to a 1100 gallon pond and ended up going as large as our space allowed to just under 5000 gallons.
There is nothing more relaxing than the sound of the waterfalls or to sit on a Summer's day, look at your plantings and watch the fish and other wildlife that is attracted to your pond.
I have found that I tend to go through a withdrawal every winter when our pond is shut down. Hubby made me some videos of the pond and put them on disk so I at least have that through the winter.
We built a lower deck the butts right up to the pond. You can see some of the deck in the pictures below. It is so nice to take a cup of coffee and sit on the lower deck on a sunny morning.
Post any questions you may have and we will help as best we can.