Water gardens can be of any size. You can grow a small garden in a whiskey barrel lined with a heavy plastic sheet. You also can find molded fiberglass ponds with or without waterfalls that will fit the scale of small gardens or patios. Medium-sized ponds may be constructed in excavations backfilled with sand and then lined with larger fiberglass preforms. Two other approaches are liners that come in sheets or kits to conform to the size and shape of pond you want.
Liners are reasonable in cost and the kits have complete instructions.
The last option for ponds is concrete. Take care to study and plan well before installation as changes are a major project if you change your mind. Whatever the size and shape of pond, decide on a suitable pump for your fountain or waterfall. A small pump will do because a minimum flow is all that is needed. Remember, this is not a jaccuzi.