Q.Petunia waves from seed
I started my petunia plants from seed indoors. Once they had their true leaves, I transplanted them into peat pots. They are now getting bigger and I am wondering if I have to transplant them into a larger pot or if I am to just leave them alone and let them do their own thing. They seem to be getting taller and have only grown a few more leaves. Do I cut them back to encourage growth width wise?

When the seedlings reach 6" in height, pinch back the main leader to promote new side shoots.
Make sure your seedlings are receiving a good amount of sunlight. At least 6 hours a day.
https://www.gardeningknowhow.com/ornamental/flowers/petunia/wave-petunia-plants.htm