Being a rank newbie myself, I am not in the best position to be making suggestions, but I am familiar with the Solsylva plans and machines.
The 13"x13" uses unsupported rails and you would not want to scale that up to a larger bed because of this.
However, David (Solsylva designer) has 2 other machines that can be built with marginal effort, but have a much greater cutting capacity. The beauty of his machines is that they don't require a cnc to cut the parts; most are stock hardware store items. I myself am building the large belt driven table by hand.
If you haven't purchased the plans, I can recommend them whole-heartedly. They are very well thought out. I have tried redesigning several aspects of the machine I am building, only to discover that David had already made the best design decision. He is also very accessable by email with any questions.
That being said, have you seen the www.buildyourcnc.com design? Not the most accurate and adjustable machine, but a VERY quick build. There are video's on the site of the entire build.
Good Luck, and post lotsa pics