You will also need a rough idea of the angle at which the vertical and horizontal parts of the stem intersect; varying this angle will change both the reach and height of the bars. They are available with an adjustable angle if you cannot decide, but these are somewhat heavy and need to have the angle adjustment well tightened for safe use. I have an adjustable stem on my loaner bike so I have another adjustment to help fit the borrower and it has been trouble free.

If you buy a stem with a removable faceplate you will avoid having to remove the bar tape and controls if you want to change the stem again.