I find that hawthorn and blackthorn walking sticks in hedgerows which are been cut and then given time to grow make very good shanks maybe not very good handles but good straight shanks for walking sticks.
Any tree like Ash, Hazel, Sycamore which have been cut level with the ground usually generate new gorwth around thre base which is usful material for making walking sticks.
I find that larger trees don't have suitable material for making walking sticks, and if there was any suitable they would be very difficult to reach with a ladder. Some of the best hazel grows from the banks of steams and rivers with there roots close or below the water level. It seem that hazel which has been growen on rocky ground produce more attractive pattern in the bark than hazel that grows on more fertile ground something as simple as a attractive bark makes a perfect walking sticks.
As for birch, sycamore, ash and rowen they all produce saplings which not only make good shanks for walking sticks but when dug up correctly provide a ready made and very strong handle from the root. The best time to cut walking sticks is in the winter when the sap in the tree is not rising. So aim for november, december or january. If you have to cut at any other time be very careful with the seasoning process, normal it takes 12 months to season when you cut in november, december and january and any other time it takes at least another 3 months and even then the ends might still split which would destory the shank for making walking sticks.
As for the size of the walking sticks you are looking for, this wil be determined by there intended purpose, Crook walking sticks and thumb walking sticks will ned to be longer than conventioinal walking sticks, a walking stick for a man may be heavier and larger in dimensions than one for a woman or a child.
A good lenght to go for is about 4 to 5 ft, you will find a use for shanks either side of these dimensions so a broader range is somewhere between 3 and 6 ft. As a diameter for your walking sticks anything from 3/4 - 1 and 1/8 inch is acceptable, the idea size is 1 inch.
The balance of a finished walking sticks can be critically affected by the extent to which the walking sticks taper over its lenght, anything that tapers to half its diameter with be badly balanced. Remember the diameter of the walking sticks will shirk a bit during the drying process, and if you intend to strip the bark that will have to be taken into account.
Do not be deterred by walking sticks that have bends in them in most cases they can be easily removed after drying. The exception is a walking sticks that have a "knuckle" (a knuckle is like a knot growing out of the side of the walking sticks that makes the stick bend) or "dogs leg" ( a dogs leg is like a dogs back leg when the walking sticks is in a S shape and can't be straightened). Unless you can cut this out leave the walking sticks, better look for another walking sticks.
If you are looking for a thumb walking sticks its better to get a strong V which is the handle. Whilst is it possible to made some corrections to the depth of the V by shaving of a bit of wood, ideally should not have to be done. If the V is to wide or narrow it is possible to fix. When cutting the thumb walking sticks do not cut the V to short its finished lenght should be about 2 1/2 -3 inch, but always left it longer when cutting to allow for spliting.
Developing a good eye for walking sticks takes time but it come and it takes practice. Althought you first walking sticks may not be the best you will find something that will do to make a start.