Garden is a GREAT map to practice on, just turn the Xeno count down to 1.
Playing against a single Xeno on a Huge map, like The Garden or Rinze 100x100 (93 Worlds), will be easier than on a Medium map. You'll have more time to conduct research and grow your empire before you bump into the Xeno. Even after that you'll have lots more room to expand, while you hold the line with defensive actions and harassing attacks. So you can delay going on the offensive until you've built a massive invasion fleet, backed by an economy powerful enough to support it in Red space without breaking a sweat.