How important or critical pot size is depends on the physical characteristics of the grow medium (and the size of the root mass of the plant being repotted or potted up - and there's a difference between the two practices. Media that hold an excessive amount of water make our choice of pot size critical; whereas coarse media that hold very little water in the large spaces between soil particles offer growers plenty of margin for error.
The easiest medium to grow in will be one that holds all or almost all water on the surface of soil particles, in the internal pores of porous particles, and at the interface where soil particles contact each other, leaving the spaces between particles full of air, top to bottom of the pot. Media comprised almost entirely of fine materials (peat, coir, compost, composted forest products, sand, .....) will hold a great deal of water in the spaces between particles. This is the water that robs plants of the oxygen they need to drive root function, resulting in roots that function poorly, or worse, fungal root infections.
The images below are what I make/use for my own purposes. The second image, the one that looks like gravel, drains well enough that there is no upper limit to the size of the pot you could use to start a seed in, and still not worry that the new plant would be over-potted. You could also unpot a plant from a pot that holds a pint of soil and move it into a pot that hold several gallons w/o concern about root issues.
Al
