Geo Art simply referrs to multiple caches creating shapes and symbols on the map. A creative way of making a series of hides into something unique. Here’s a collection of examples:
Ragnild Trail, Södertälje, Sweden:
No Clue Series, Palmdale, USA:
Peace, Seattle, USA:
GCSE Logo, Sweden:
Titantus Giganteus, Bottnaryd, Sweden:
This highly creative travel bug can only be logged if you log all the caches in the series.
200 Thunderbird, Idaho, USA:
Awesome Geocaching 
You should check out “4-4-0 steam engine” series in Idaho, USA 1000 caches to create this wonderful steam engine. Starts with GC3FA3B
Is there any ruling regarding to its size besides the distances between each cache being a minimum of 528 feet?
Here is the locomotive: http://www.geocaching.com/map/default.aspx?lat=42.939433&lng=-116.21175#?ll=42.90389,-116.13682&z=12
And a big smiley: http://www.geocaching.com/map/default.aspx?lat=33.99803&lng=-98.63749&z=13#?ll=33.99803,-98.63749&z=13
REIMS FRANCE : https://www.geocaching.com/map/default.aspx#?ll=49.265457,4.03125&z=12