Part of the reason for less sharing of designs, is that properly designed timber frames are less easy to modify and adapt than stick frames. If your stud spacing is 400mm O/C then extending a wall is as simply as adding more studs following that pattern. A large opening in a timber frame may be engineered for a specific species and loading. Increasing the span, or changing the timber could take you past the specified loading and risk a collapse of the structure.
Secondly, it's a more specialised skill. The standard tables that can be used for stud and joist spacing don't apply which means that each structure has to be engineered. The maths isn't overly complex, but does go a little beyond simply reading values from a table.
Finally there's a smaller number of designers out there. When researching construction methods for our house I found hundreds of stick frame designs, but it's worth noting that few of the authors of these designs had actually built them, or any other frames.
Timber Framing for the Rest of Us is a good book, but won't get you the ability to use smaller timber. It just replaces the traditional pegged mortise and tenon joints with simple bolted lap joints. You'll still need large timbers to span large areas. Alternatively you could look at a Colombage (French) style construction which makes extensive use of smaller timbers. This may however not fit the aesthestic that you seek and this will be the final problem. If you're just trying to save money, look at stick framing (or roundwood framing, search for Ben Law). If you want that look of open plan spaces bounded by large timbers, then you have no choice.
