What is the standard stair riser height?

They are the height it needs to be in order to make the desired number of treads connect the low and high points. That being said, Stair risers should be no more than 8" and treads should be no less than 11". Once your risers start being taller than your tread is long, you are building a ladder, not a stair. If you start at 8" for a riser and 11" as a tread, By shrinking the riser and adjusting the treads to fit you can get the steps to work out so that one consistent riser height can be achieved. One that will be in the 6-8" range. If you use risers that are less than 6", you should lengthen the treads. Example problem--

Lower floor is 6 feet lower than the higher floor. Keep in mind you always have 1 more riser than tread.

6'-0"= 72"

You can make 18 each 4" risers (72"H /4"R=18 risers) and have a run of 34 feet using 24" treads.

or

you can make 12 each 6" risers and have a run of 11 feet using 12" treads.

or

you can make 9 each 8" risers and have a run of 7'-4" or 88" using 11" treads.

You will always have at least one "given"-the height you need to traverse. From there it is just division and multiplication based on the 8" maximum riser and the 11" minimum tread.