No one has answered, and presumably you are past this point anyway, but I will comment.

You need to talk to experts involved in the business....to see how compact the unit is...our unit is fairly small, but the air ducts coming off the unit are not...they take up a lot of space.

When you say DIY, I assuming you want to put a horizontal system in where you have to dig all the pipes in below frost level. I suppose if you're digging anyway, it's possible, but do speak with the experts about how deep and how much area you would need to cover.

We went with a vertical where they drilled two 250' wells in one day, then came back the next day to put in the piping and the bentonite. It went very quickly and easily. And since the wells are 4' underground, they are completely unseen from the outside. The system has worked very well for us for the past 2 1/2 years.

Design and install of a geothermal loopfield is crucial to working correctly.
I am a drilling company from southern Nj and install geothermal loop fields. We have installed a bit of them over the past few years. Digging trenches is quite easy for a DIYer, but if not deep enough or designed incorrectly it WILL cost you more in operating cost, and running cost due to low heat exchange rates.