How can I size the components of a square wave generator in order to obtain a 7kHz frequency?
What you're making here is called a relaxation oscillator.

Here's a reference for that: http://www.analogzoo.com/2015/01/relaxation-oscillator-design/

The summary for a basic calculation:


Charge / discharge time is the time constant which is 1.1*RC.
Period is twice that, or 2.2*RC.
For frequency we get 1/(2.2*RC)


Some items to check and/or fix in your design:


1uf and 68 ohm is kind of a low impedance for an op-amp to drive at that frequency. Scale these to less-demanding, smaller-current values (more below.)
R1 and V1 are not needed if your intention is a square wave that swings from +VPS to -VPS. You need to refer the Schmitt feedback only to 0V.
If you want to affect duty cycle, refer R2 to a voltage instead of GND. Note however this will also affect frequency.
The LM358 is kind of slow, which will affect the charge/discharge time (it will be longer.) A faster op-amp will give closer to predicted results.
You can compensate for the slower op-amp by reducing the value of R4.


With this in mind, we can select 0.01uF and about 6.5K to get 7Khz with an 'ideal' op-amp. Try it here.