A pinometer is the standard unit of measuring distance in Viva Piñata. On your Pinatas info page you can see how many pinometers it has walked since becoming a resident. Square Pinometers are used in measuring an amount of something that you have (e.g. grass). Having a large enough area is part of the requirements for a piñata to become a resident, have a romance dance, or stay healthy. (Crowded piñatas will become ill if they don't have enough room to roam.)
One section of fence is equal to one pinometer. The largest garden is 32 pinometers on each side which makes for 1024 square pinometers total.
10 square pinometers (1% of the largest garden) is an area just over three fence lengths on each side.