If my seedlings are showing at the drainage holes, they are ready to transplant up, unless they are going into dormancy, in which case I would wait until new growth starts in the spring. Al

Does it have anything to do with the height of the seedling? It said in Park Seed's manual that it's best to transplant seedlings outdoor when they are at least 4" tall. Is there any truth to this?

You do not mention what seeds you working with, or what zone you live in. You first mentioned repotting to a larger pot, now you are talking of planting in the garden. Please give us more information so we can help. Al

Asking about transplanting to garden is only theoretical. I am working with seedlings of Coreopsis, Salvia, Purple Palace, Bellflower, Lewisia and PowWow. I live in zone 6a and I was late in sowing the seeds in time to transplant them outside; so I am going to grow these seedlings to a height where they are suitable for transplanting outdoors into the ground (next spring) and when they have reached that height, I am going to place the seedlings in my unheated garage to overwinter them and hope that they go into dormancy. And so when spring comes, I will bring them inside, and once growth resumes, I will transplant them outside.

Please let me know if there is anything wrong with how I want to proceed with things. And also answer my prior question. Thank you.

It is normal to transplant when you have two or more true leaves, using height of the plant as a guide is not good practice as there is too much variance with plant varieties. I can not indorse what you have in mind regarding planting now for next years planting. You could have, a couple of months ago outside and had enough growth established to survive the winter, at least for those perennials hardy where you live. You would be better off starting your seeds inside for spring planting, so you could plant out side without a check in their growth. To seed now and keep over winter would produce sick plants for spring planting. Al

It depends on WHAT vegetable/plant we are talking about and what is the original size in which they were sown.