Trimming smaller than the recommended trim lines is one of those things where "if you have to ask, its not for you." The more you cut off, the more things you have to troubleshoot, and the more difficult your soldering points become. If you don't have experience troubleshooting the Wii or navigating the compendium, you should stick to the lines in the guides. Once you develop the experience to troubleshoot an unorthodox trim, you will know exactly where to cut.
Speaking from personal experience, building a compact portable is substantially harder than building a larger one, and will require a lot more material cost than the price of a Gboy kit.