Don't want to derail this photo thread, but, for a discussion on choosing a pilot chute size, I did a write up a while back in this post:
https://www.mountainmanrockets.com/index.php/builds/8-inch-agm-33-pike-build/ (in the table of contents, jump down to "Choosing a pilot chute")
Here is the abbreviated version:
For these calculations, I used the military’s Parachute Recovery Systems Manual –
https://apps.dtic.mil/sti/citations/ADA247666
For this specific discussion, let's assume we want to extract a Cert 3 XXL (which weighs about 4.5 lbs with dep bag). We need to look at the reference of pilot chute selection beginning on page 267 of the PDF. This discussion assumes you have read much of the preceding 266 pages, but, in short, it recommends a pilot chute have an extraction force of 4 times the weight of the main parachute.
The manual goes into some detailed math to calculate some of these figures, but we can assume much of what we need to worry about is going to be simple, near ground level (as opposed to calculating sub-space and high altitude forces) and nominal weather.
We need to know the dynamic pressure for the day in question, but let’s assume a normal temperature (60-70 degrees) and a generally normal weather front in the area. For this we use Bernoulli’s equation. If we are using MPH as our unit of measure, then dynamic pressure (q) for a normal day on the East Coast would be:
q = v
2 / 391.2
This formula can be found on page 62 of the manual.
If we assume the rocket will be falling at about 90 MPH under drogue, the dynamic pressure would be:
q = 8100/391.2 = 20.7 lb/ft
2
We can then then use this to figure out the Drag in pounds for the parachute using the formula:
D = q x S x Cd
Where S is the surface area of the chute (in square feet) and Cd is the drag coefficient.
The area for a 36″ chute is 7 sq ft and the Cd for a Fruity Chutes Iris is 2.2
So total drag for a 36″ Iris Ultra Compact Fruity Chute is:
D = 20.7 x 7 x 2.2 = 318.78 pounds
The Cert 3 XXL parachute weighs 4.5 pounds, so the rule of thumb the military would use is that it needs a pilot chute with a total extraction force of 18 pounds. This shows that a 36″ Fruity Chute parachute, with 318 pounds of drag capability (on a nominal weather day at sea level at ~90 MPH), would be more than sufficient as a pilot chute.
