A pond of any size will benefit some form of wildlife. The water you provide may be a vital stepping stone linking up other larger ponds in your neighbourhood, or just provide some much needed drinking water for a thirsty bee or hedgehog.
Create a bog garden by burying some cheap pond liner about 30 cm down. By piercing the liner a couple of times you will allow some drainage but will still keep the soil above wet. Bog gardens provide a great habitat for toads, hoverflies and many other invertebrates.
Small ponds can still benefit amphibians by providing a place where they can cool off in the summer. Practically anything could be used to make a small tub pond: an old sink, a bucket buried into the ground or even an old paddling pool. Add some native pond plants to make sure the water is full of oxygen and remember to make sure the wildlife can get in and out easily by creating ramps of stones, pebbles and logs.