Hello
I’m sorry to see, but it looks like small pests called thrips have taken over the leaves, and are eating the plant. The easiest way to get rid of them is to toss the plants and substrate. More info about them in this thread.
And to answer your question…
There is no right or wrong, but the usual recommendation is no more than 2" larger in diameter than the plant itself.
Otherwise, it could be too big for the plant, and therefore possibility to overwater the substrate.
Some tips:
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Make sure you water the plant pot enough during the first few weeks - the soil should be moist but not too wet. The water helps the plant settle into its new home.
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The pot shouldn’t be considerably larger than the plant pod. A pot with a 6 inch (15 cm) diameter is fine.
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If you’ve repotted a perennial (eg. thyme, lemon balm, tomato, chili pepper), we recommend repotting it each year in a pot that is +1 inch (2.5 cm) bigger than the previous one.
To rule out root-bound it’s suggested to loosen up the roots and cut them a little bit before repoting them.
Hope you’ll get rid of thrips easily and keep growing plants!