I can attest from personal experience that if you build it, they will come. Just not necessarily who you might expect...
Practically speaking the smaller the pond the more maintenance you have to do. Some of the factors that are in play:
- small ponds heat up and cool down faster due a smaller volume of water
- small ponds tend to go "green" with algae faster in the summer
- all the same requirements of a larger size pond: moving water, a pump, an electrical source
- In colder climates the shallower the pond the more likely it will freeze solid killing most fish and some amphibians
- they do not require water to drink, only to breed
- a suitable pond will have shallow edges to make it easy to get into to. Half barrels or raised containers are not suitable
- when toads or frogs lay eggs they do not protect them. If you have fish in the pond, like a goldfish, it will eat the eggs before they spawn.
If you are very tight on space and would like to have something enticing for animals you could do so with a seasonal pond or vernal pool.
- Find a low area in your garden
- dig out shallow area, say thirty to sixty cm deep
- line with EPDM food grade pond liner, 60 millimeter thick is preferred, 45 mil will do
- edge with flat stone. For a leak free experience place one layer of stone on the edge of the liner and then fold the liner over on top. Place a second layer of stone on top to hide the liner. The "U" shape will prevent water loss by capillary action.
- divert downspout water into this area with piping or shallow trenches
- fill partway with lava rock(often sold for barbeques) or bio balls
- then put some landscape fabric down and a mixture of organic matter and sphagnum moss
- plant with bog plants or other water loving plants
By doing this you will have an open water area in the spring when amphibians need somewhere to lay eggs. If it dries out in the summer, no problems and no home for mosquitoes
Edit: corrected recommended liner thickness to millimeter. Thanks @jmusser