Control key frame value with slider control
You were close.

if (numKeys &gt; 1){
  t1 = key(1).time;
  t2 = key(2).time;
  v1 = [95, 95]; //or if it's 3d [95, 95, 95]
  slider = comp("INPUT").layer("CONTROL").effect("Zoom intensity")("Slider");
  v2 = [slider, slider]; // or [slider,slider,slider] if 3D
  linear((time, t1, t2, v1, v2);
} else {
  value
}


You were right that the scale property requires 2 (or 3) dimensions. So you need to use arrays, which look like [value1, value2], or [value1, value2, value3,… valueN] for as many members you want the array to have.


On line 4 I assume you want it to be 95% not 0. I also changed it to an array.
on line 5 I assign the slider value to a temporary variable and then put that in an array. You could do it in one line by repeating the comp("INPUT" … (Slider) phrase inside an array, but it would be really long and awkward. Alternatively you could use a point control if you wanted to be able to set the dimensions separately.
on line 8 the else without curly braces works, but it makes me nervous.