If you got 10 land kiters together and asked them 'what kite should I buy first' something like 7 out of 10 would usually say 'a 3m 4 line low aspect power kite on handles'.
Why 3m? Power kites are generally made in sizes from 1.5m to 21m. You want something that will be easy to learn on, still provide some decent pull but won't completely wear you out. Something around the 3m mark is usually ideal for the average weight person. For a start, you don't want to be fighting a kite while y