Si Nguyen
Omono
They look to be about 4-5' tall and am thinking of chopping them down to 2.5', then excavate from there.
When you chop, before you start to dig, try to leave a long section on the trunk to use as a handle bar to pull the tree out with. Faster this way. Then when the tree is out, saw off the handle bar of the trunk to your final desired height. Your final height should be as low as possible so you won't have to chop it again later. Try to chop it only once. People tend to make the mistake of chopping the tree again too soon, like in a few months or a few years, when they finally see the bonsai in it. Chopping it again too soon can kill the tree.
Good luck!
Si