Micro571, apparently you didn't do your research. The Micro ATX form factor is 5.9" X 5.9" That's considered a full motherboard solution for the Micro ATX form factor, there are multiple hundreds of products out there, they're often called shoebox PC's (for vertically mounted add on cards). For under 100 dollars, you're pushing the boundaries of sanity for a full fledged computing device. You can find smaller and better for a LOT more, but that's because they're niche markets, the economy of scale isn't there. For cheaper you have to go much lower power, so far much lower that you should re-think your criteria and design goals, to double your budget you'll increase the power by an order of magnitude or two... The cost/benefit ratio is so staggeringly in favor of avoiding dirt cheap it's not even funny.
Just think what the cost/benefit will be when you get the system you set your design goals on and find out that you want to add a feature that the system doesn't posses. You'll have saved 50-100 dollars at the gate! For a product that will fail to reach your goals, a brick, or it'll do what it's always done just fine and you'll have to go buy a whole new product to meet your new goals. You have to think ahead.