Squeaker,

I agree with Tinylake, the greater the volume of ground contacted = the greater heat exchange potential.

Is your trench length limited by the boundaries of your property? Are you digging the trench, or a contractor? Either way, there is usually a minimum hours charge. If you have a four-hour rental, dig "four hours" of trench and space the coils to the maximum trench length. (You do need to allot some time to push your spoils back over the trench.)

Also, are you digging in soil with a decent clay content? I have some sandy soils on my farm that are impossible to trench - the sides will immediately slough into the excavation. OTOH, I have clay-rich soils that will stay open with perfectly vertical sides all of the way to total depth.

I suggest you read up on some ground-loop geothermal systems for houses in your area. There should be some info on best practices, sizing the system, pump rates, etc. if you dig long enough. (Pardon the pun, I only caught that when I previewed my post.)