For the purpose of this help page, we can only offer our opinion.
First, one must consider some of the variables; your height, experience, likely terrain, tree stock size or type, & contract specs. Since you may not know the stock size or contract specs at this time, your initail choice might be based on height & experience.
SHOVEL LENGTH
To be your most productive, you would use a shovel that is appropriate to a given ground type. For example, loose easy ground is fastest with a super short shovel. Tougher, more technical ground (grass mat) would require more leverage from a longer shovel. So, you can see that there really isn't a silver bullet. The closest you get to the silver bullet, is a "Staff" handle.
For the average "treeplanting" ground type found in Canada, you'd want your treeplanting shovel to be porportionate in length to picture 3. Your shovel should never be long enough to bend your elbow like in pic 1.
EASY GROUND
Really, the shorter the better; but a commonly heard rule is, while standing straight up, hanging your shovel down with your arm straight, your blade tip shouldn't touch the ground. So, you'd simply measure from your palm down. See pics 2 & 3.
TOUGH GROUND
You want your shovel as long as possible before it bends your elbow. Now, this may sound close to your other shovel, but 3+ inches or so can provide enough of a difference to warrant a second length.
STAFF
If you think that you're going to run into a wide variety of variables, consider using a staff handle. A staff is never too long, and never too short, because your hand goes where it needs to. If you don't like staffs, you can always add to your quiver.
WORKWIZER COMPONENT SPECS
Workwizer "shovels" are measured from the top of the grip to the tip of the blade, to give an overall length.
Workwizer "handles" are measured for usefull length - from the top of the grip to where the shaft meets the blade.
Workwizer "shovel blade" measurements only include the working surface of the "spade", and do not include the "socket" or "stem" - add 9" to include the socket...
HAPPY PLANTING