What's the difference between large frame and samll frame Tai Chi?
As I understand it, the size of the frame refers
to how large or small the movements are. That is,
large frame has movements that use a larger
range of motion than small frame. So small frame
movements will seem tighter, faster, because the
radius of the circles is smaller. Some people
say that small frame is closer to what a movement
will look like when applied. Keep in mind: you
can always go from larger to smaller, but it is
harder to go from smaller to larger.
Chen style has large frame and small frame. Both Hao style and Wu style (Wu Chien Chuan style) are considered to be small frame. Yang style has small, medium and large frames. Small frame is attributed to Yang Pan Hou, medium frame to Yang Chien Hou, and large frame to Yang Cheng Fu. Chu Gin Soon of Boston apparently teaches medium frame Yang style. See article in Volume 11, number 1 of Journal of Asian Martial Arts for a comparison of medium frame and large frame Yang styles.
The large/small frame distinction is one I have often heard in Yang circles. In the U.S. the popular Cheng Man Ching short form is more compact and is considered the "small frame" of the parent Yang style.
There is also a range of extensions within the styles. Chen and Yang tend to have the largest arm and leg extensions. I practice Wu Style (from Wu Chien Chuan). It would be considered a "medium frame" style with more compact positions (compare the Single Whip posture as an obvious example) . Hao ("the other Wu") style is a "small frame" style with the fingers-spread guard position held very close to the torso.
Everything evolved from large movement Shaolin style Kung Fu. In Chen (the oldest major style), more "external" and obvious applications are present. Yang took out a lot of the ballistic and shaking/pounding motions, but still retained the full extensions. Wu and Hao refined this even further and made the movements more covert. For example, many applications of elbow techniques are not obvious in the Wu form and are introduced later in pushing hands training. Basically the smaller the style, the more reliance there is on generating movements efficiently from the body core. In this sense the newest Wu and Hao styles are the most "internal". You could say that they are more updated and energetically/ergonomically correct.
Caveat: There is always ongoing tension between those who want to preserve authentic/traditional methods, and those who push to improve and refine them. Before anyone starts a style vs. style flame war, I want to be clear - there are excellent martial practitioners in all major Tai Chi styles. Access to a good teacher, and your own training are much more important than the style that you choose. It is o.k. to contrast and compare them, but since they all have different training techniques and progressions, I do not believe that a mix-and-match approach will be very productive. If you can find an authentic source, stick to a single style and go deep.