The literature I see suggests potting up to 3 or 4 inch pots. Last year I used 4 inch pots but most became root bound because I delayed planting out due to wet and cold weather. Are there any reasons to avoid using larger pots such as 5 or 6 inches to pot up?

The reason for me to avoid large pots is the space required for storage. I can get compost loads for the cost of gas money, so mix prices wouldn't play into it. Even still, I don't pot up, for the most part, at all. I start seed in 2" soil blocks, 50 per 1020 tray, and they stay there until they are 6" or so, and then get planted. Our planting time here is mid May, so I usually delay seed start until mid April. They grow very quick in the warm May air. It becomes a lot of work/space management, not having a real greenhouse. When I do pot up, it is to delay planting due to the planting spot not ready - not due to cold weather. Then it is into gallon milk jug bottoms, with two plants trenched in sideways to begin growing more roots. It just takes up so much space, but they are really strong plants that way, and at least the milk jug bottoms are square so they nestle together.

With small plants, I think they thrive with a little bumping. Gives them some shade relief on a too bright day, and keeps them from getting broken by heavy winds. They might tangle some but are more supported. With 6" pots, they are pretty much on their own.

I agree, the only reason not to use larger pots is the space required. I like to use beer cups for the smaller footprint cw a 4 inch square. But people with more space sometimes use large pots and take more time to grow the plants bigger before planting.
Of course it is optimal to plant them out as soon as they are getting well rooted in whichever size is used, but we all have years when the weather disagrees with those plans. I find the biggest problem with plants that had to stay inside a bit longer, is that they aren't getting the light they wanted because they're increasingly crowded and shading each other. I never worried about them filling the cup with roots. Mine are usually chock full of roots when I plant them out, and if anything it seems to reduce transplant stress. More weakly rooted specimens take it a little hard.

If transplanting cucumbers and melons it's always best to let the plant get root bound and not disturb the roots.
Vines about 18 inches long or so.
It not it's almost a guaranteed failure.
Go from 4 inch pot directly into to ground.
Make mold of small containers in new soil and place plant in hole and water.

This is why I always hold myself back from planting too early. My seed starting date is at the end of March.

Whenever I have decided that "it's getting warm early this year" and planted earlier, I always regretted it. Something always happens like an unexpected cold snap in May, serious wind or really wet weather and planting out gets delayed leading to pot bound, tall plants.

I only pot up in 4" pots due to space concerns and I do not want to purchase lots of soil for 25 big containers either. Garden soil is too heavy for pots.

Once my plants are big enough to pot up they no longer fit under my lights and they get to live on the front porch which they like better anyway.

20180429_084157 by Brownrexx, on Flickr


Seedlings 2021 by Brownrexx, on Flickr

I pot up using 2"X2"X3" deep plastic pots. Always has done well by me.