An often asked question is when to plant out seedlings. A lot depends on what seedlings you want to plant and the current environment. Such as some current daytime temps in your garden may or may not be ideal for some seedlings.
But another issue is size of seedlings. Does it matter what size of the seedlings before planting them? To this I can but give my very best opinion based on years of experience: Well…. It depends.
And that depends on many factors.
If you sown them early, have a warm greenhouse, or grow lights, and the space, then the larger you can get your seedlings the better. Because that means a longer in ground garden time and maybe a longer harvest time for plants that you progressively harvest, such as lettuce. And larger starters can be more resistant to total devastation in one night from slugs.
But on the other hand…. historically most plants are grown on site from seeds, so they will be tiny and rather helpless for a long time in the garden anyway. So planting small starters will give you a few weeks head start. But otherwise if you need to plant them small, do it. Especially if you have limited space for the next set of starters. It should not be a problem much beyond direct sowing. The entire point of sowing, and even needing to plant even early smaller starters, does still have additional benefits beyond direct sowing. Such as avoiding thinning, a few weeks head start, options for inter plant starting, neater placed planting and densities, etc, etc. So I do it and thus recommend it. Even if I sometimes plant rather small starters at times as needed based on may starter space, which is limited.
So again, it depends. It depends on what you want to grow, how you want to grow it, how you need to grow, your limits, and needs, etc. etc. etc.
Gardening is based on collective information, but also on your personal experience with your garden. And one needs to take the time to learn how to best garden your garden.
Hope this helps.