Gar rarely get to the maximum sizes known. For example, the Florida gar has a maximum size of 1.3 metres but most adults are 33-86 cm long, according to the
Florida Museum of Natural History. Most aquarium specimens seem to get to around the 60 cm mark. It also seems
probable that females get bigger than males because they live for up to twice as long, so if you want a smaller fish, pick a male.
I believe, but I'm not sure, that Florida gar,
Lepisosteus platyrhincus are currently the species most often seen in the UK. They don't range into the cool temperate states of the US, so I wouldn't keep them in a pond all year round. The cool-water species are
L. osseus, the long-nose gar, and
L. oculatus, the spotted gar.
I looked after what I think was a spotted gar in a 200 gallon tank. In the three years I was looking after it, it went from about 40 cm to 60 cm. It mostly ate bits of squid and mackerel but also Hikari "cichlid gold" pellets. They don't need live feeder fish, and indeed some populations (like the brackish water Florida gars) apparently feed primarily on swimming crabs. Gar basically eat anything, from ducks to carrion.
A good place for general gar fun is
Save the Gar, a slightly odd name for a site primarily about gar fishing, but there you go.
Cheers,
Neale