There are indeed some great handles shown here. I love DaVid's sculptured handles. DCM's handles are all very elegant, functional and well finished. Manny's stuff is also well executed and looks great with very simple materials. I hesitate to post any pictures of my handles, these handles by no means show the refinement of a lot of the others posted, but will post for the sake of showing the proportions of working handles.
The majority of the handles already shown are rigid and essentially non-working. I have come to favor working handles lately to share the some of load with the bow arms.
Here are two handles from paddle bows I have made. These are bare with no handle wrap. In the final form I usually wind the handle with thin leather "lace" leather "string".
Top is 54# @ 26" oak bow 58' long.
Bottom is 43# @ 25" ash bow 54" long.
Both are about 1.25" wide x 0.75" thick at the handle center.
Both are working with about 80% of the strain level of the main bow arms. (I like to have the handle work slightly less that the bow arms as a little safety margin)
Here is a picture of a similar bow, (41 at 25 , about 50 at 28) with a temporary leather wrap. The small black dot in the center indicates the bow center, and the large dot on the side indicates the arrow pass which is a bit high for me but matches style of the shooter of this bow. This handle is also 0.75" thick by about 1.25" wide, bow is 58" long.
Next is a modification to a "long bow" handle, originally 1.5" wide, and 0.75" thick. I modified it to 0.87" thick by 1.02"wide resulting in similar strain levels to the original, but a narrower, and much more pleasant to shoot bow. (46# at 25", 70.5" long) It only took 0.12" (3mm) more thickness laminated on the belly side to get an equally strained handle that is only 2/3 as wide as the original. The extra thickness means less bend (by the ratio of the thickness change) than before for the same strain, raising the draw weight by a little (46# vs 42#), and slightly increasing the strain in the arms.
( Please ignore the messy bench in the background. )
Here is the final bow showing side and front views of the handle.
Lastly here are a number of charts for the thickness and width of fully working handles for a number of bow woods. The top most line on the charts frequently represents a handle that is thicker than it is wide. This is an unstable condition and is shown only for seeing the relationship of width and thickness. In general, never build a working handle that is thicker than it is wide! Notice also that the handle dimensions are a strong function of the length of the bow: longer bows need stronger handles.
These charts are computed based on 50#, and 28" draw, with about a 6" brace. For higher weights, increase the width proportional to the intended weight. For lower weights decrease the width proportional to the decreased weight, (remembering not to with width less than thickness). For other draw lengths scale the whole bow up or down proportional to the intended draw length (length, thickness, ) then adjust the width to restore the weight.
Although not very artistic, these dimensions are sort of a minimum limit for handle dimensions for bows (depends on your wood of course). You can always go thicker, and wider for a stiff handle bow, but going much smaller than these dimensions will make the handle the weak point of the bow, and risk breakage right at the handle.