Pot size affects a plant far more than new container gardeners usually expect, and getting it wrong in either direction causes real problems. Too small, and roots quickly fill the available soil and become root-bound, restricting further growth, and the smaller soil volume dries out much faster in the sun, meaning more frequent watering just to keep the plant alive. Too large, and the extra soil holds more moisture than a small plant's roots can actually use, which keeps the mix wet for longer after each watering and raises the risk of root rot, particularly over a damp Auckland or Wellington winter.
As a rough starting point by plant type: shallow-rooted herbs and salad greens like lettuce, basil and parsley do fine in fairly shallow pots, commonly in the 15-20cm depth range, since their root systems don't run deep. Fruiting crops need considerably more room. This site's own plant data gives tomatoes 50cm of spacing and capsicums 45cm when grown in the ground, which reflects how much root space they actually want; in a container, that translates to a genuinely large pot, commonly cited at 40-45 litres or more for a tomato to crop well, not the modest 10-20L pot new gardeners often start with. Undersized pots are one of the most common reasons a container tomato produces a handful of small fruit instead of a proper crop.
Undersized pots are one of the most common reasons a container tomato produces a handful of small fruit instead of a proper crop.
"Potting up" is the practical middle path between these two mistakes, especially for seedlings and young plants. Rather than moving a small seedling straight into its final large pot, it goes into a pot suited to its current root size, then gets moved up to progressively larger pots as it grows. Planting straight into an oversized pot from the start doesn't give the plant a head start; it just means far more soil than the roots can use, which stays wet longer and is more likely to cause problems than help.
Depth matters as much as width for some crops specifically. Root vegetables need real depth to develop properly: carrots and other long root crops need a pot deep enough for the root to extend without curling or forking, and a wide, shallow pot that would suit lettuce perfectly is a poor choice for carrots even if the total soil volume looks similar. Always check that a pot has proper drainage holes as well; a pot that looks the right size but holds water at the base defeats the purpose regardless of how well the dimensions match the plant.
When in doubt, sizing up slightly rather than down is the safer mistake for anything you intend to keep growing for a full season, since a plant that outgrows its pot mid-season is a bigger setback than a slightly oversized pot early on. For anything short-lived, like a single crop of lettuce or a herb you'll replace each season, matching the pot closely to the plant's actual root size is the more efficient choice.