What pipe length should I use for my mini split AC/heat pump?

Question

I just bought a 7000 BTU mini split AC/heat pump and it didn't come with the installation pipes. The seller offer pipes of 3 ft/6 ft/9 ft etc. for the AC. Should I get the 3 feet pipes and hang the outdoor unit above ground next to the indoor unit, or should I get longer pipes so that I can put the outdoor unit on the ground? Thanks.