Shawn, actually, I'm in the final stages of building one of these. Although they aren't exactly "portable" like a woodmizer without more modification. I got the plans and pictures ~$50 from Bill Reeks in KY.
I had a bunch of scrap 2"x4" channel that I used for the frame and since I'm taking a stick welding class this semester from the local tech college, I thought this would be a good project. The instructor is pretty excited also. I've got the frame made and I'm finishing up the platform that the saw wheels and engine ride on. I've had to fabricate a few parts and that's been a little more time consuming.
I got a new 12hp engine off ebay for $300 and with buying some other metal and parts, I've got close to $500 in it so far(not including the saw blades yet).
It's a pretty basic system, the welding is not too difficult for someone with a little experience. Bill suggest that if anything to "overbuild" your own saw and that's what I did. Very little 1"x1" tubing on mine, lots of "2x4" heavy channel.
As far as the size of the log, it's up to the length of the track you build. As far as the size of the log, it's up to you how far you space your tires apart and the diameter of the tires(length of the saw blade). I'm following most of the dimensions off the plans.
I'll take some pictures after work today and see if I can post a few tomorrow.
In the meantime, if you have any specific questions/w3tcompact/icons/hmm.gif fire away and I'll try to give you my answer.
gary