There are many questions that come from a person that is considering putting in a water feature.  The first is usually "How much will it cost?".  Yes this is important obviously, but what is more important and to some extent changes the answer is how large you want your feature to be.  Whether we are speaking of large water gardens or a patio fountain, the answer you will probably get from me is, "As large as possible".  Now you may think that is a monetarily motivated answer, it isn't, it is a maintenance motivated answer. 

A fountain's size, and more importantly the basin's size, is directly responsible for the maintenance.  Outside of occasional algae and lime treatments, refilling the basin is the lion's share of the maintenance.  The larger volume of water you have the less often you have to re-fill it.  Also as your fountains gets taller the basin needs to get wider in order to catch the splash from the plume or wind.  Basically, the larger the basin the less maintenance.

A water garden's size is also relative to the maintenance you will experience, however not in the way you are probably thinking.  Yes larger ponds can lead to more maintenance, but if it is fitted with proper hardware, bio load, and plants, the larger ponds are less maintenance than pre-forms or small EPDM ponds.  This may seem counter-intuitive, but ask those people who have a small pond how much time they spend trying to keep it clear, it will probably be similar to a well fitted large pond.  The more volume of water you have the more consistent it stays, fluctuations cause algae issues and many other problems.  Now keep this within reasonable bounds, but that extra volume will help keep things balanced and minimize maintenance.  (Also if you have fish it will cut down on predators if it is at least 7' at the narrowest point and at least 2' deep.)

Of course a larger pond or fountain costs more than a small one, but making something a little bit bigger is a lot less expensive than doing it over.  Also a lot of the cost is getting the crew there and machinery use, it shouldn't cost twice as much to double the size of a basin or enlarge the body of the pond. 

Finally, while bigger is better in most cases, keep it all relative to your setting.  A 20 foot fountain next to a 15 foot house isn't going to work any better than a 150 gallon pre-form in a 5 acre yard.  Make sure everything blends and fits well with its surroundings before you pick up a shovel or hire a contractor.