I have a curve in Animation Editor:



How could I force the handles to stay the same length? I tried "Tie Handles" but it only forces them to align with each other.

The next button along, "Un-tie" handles

EZiniT
The next button along, "Un-tie" handles

Not sure what you're trying to say. Untying handles won't force them to be the same length (of course not!)

you can enable Keep Acceleration Ratio {R} button to adjust both handle lengths at the same time

to force them to be the same length first, you can maybe select them both and set the same accel value

The first clip is what I thought you were asking but in hindsight....I think I get you now ^^

How I got equal lengths was hit the "tie handle" option to get a straight control as in your image where both
have differing lengths, select only the first "tie point" copy the Accel value from the bottom of that panel
into the second tie handle.....this will make the lengths equal (I think)

To adjust the curve from there turn on "Untie Values" not "Untie Handles" and select the actual "line" linking the key to the "tie handle" to rotate the handle around the key maintaining length.

I see. Thank for both of your answers!