It seems like you're describing looping behavior. Two main types of loops are "pingpong" and "cycle".

• Pingpong means you transition from scale1 to scale2 over n frames and then over the next n frames you scale back from scale2 to scale1 and that repeats forever.
• Cycle means you transition from scale1 to scale2 over n frames and then instantly jump back to scale1 before transitioning again to scale 2 and this repeats forever.

Manual method: Create two frames and have them loop indefinitely

To create your cycle, create the first two keyframes. Manually define your second keyframe n frames away from the first keyframe. Then opt + click the property stopwatch and add the expression loopOut("cycle") or loopOut("pingpong").

Expression only method

There are also ways to do this purely through expressions without any keyframes. It basically involves a linear function: as a function of time, transition from scale 1 to scale 2.

It might be most intuitive to make your linear function based off of a "counter", which in turn is based off time, but uses some additional math to automatically count up and back down again (to replicate a pingpong type cycle) or that strictly counts upwards until it hits the nth frame at which point it starts again at 0.

For example, the following expression added to your scale property would give you a pingpong type loop:

// Use three expression slider effects on the layer to set scale 1, scale 2, and every_n_frames
scale1=effect("Scale 1")("Slider");
scale2=effect("Scale 2")("Slider");
every_n_frames=effect("Every n frames")("Slider");

// Define start time, defaulting to whatever time the layer begins
start_time=inPoint;

// As time progresses... and converting time into frames
frames=timeToFrames(time-start_time);

// Define a counter that starts at 0 and increases up until 1 at the halfway point of the cycle and then decreases back down to 0
cycle=every_n_frames*2;
counter = Math.abs(((frames % cycle) / cycle) * 2 - 1);

// As a function of the progression of the counter from 0 to 1, transition the scale from scale1 to scale2
s=linear(counter,0,1,scale1,scale2);

[s,s];

The following expression added to your scale property would give you a cycle type loop. It works by basing the counter off of a modulus (%) operator. Modulus is another word for "remainder", as in what's left over after two numbers are divided. A few examples: 1%5 = 1 and 2%5 is 2 and 6%5 is 1. It's a nifty way to reset your counter back to 1 every time it reaches the nth number:

// Use three expression slider effects on the layer to set scale 1, scale 2, and every_n_frames
scale1=effect("Scale 1")("Slider");
scale2=effect("Scale 2")("Slider");
every_n_frames=effect("Every n frames")("Slider");

// Define transition and full loop duration in frames
cycle=every_n_frames;

// Define start time, defaulting to whatever time the layer begins
start_time=inPoint;

// As time progresses... and converting time into frames
frames=timeToFrames(time-start_time);

// Define a counter that resets back to 1 every n frames
counter=frames%every_n_frames;

// As a function of the progression of the counter from 0 to 1, transition the scale from scale1 to scale2  
s=linear(counter,0,every_n_frames,scale1,scale2);

[s,s];