At some point in the growth of your plants, you will have to consider transplanting the plants (into bigger containers). Here are a few basic guidelines that have worked well for me.
As a general rule, I think a plant is ready to be transplanted when it's about 2-3 times as tall as the container it's in (assuming the container is nearly filled to the top). If a plant has been in the same small container for several weeks, it's a good bet it's ready to be transplanted into a bigger pot.
The exception to my rule seems to be plants that do not stand up on their own and that need support. These often include bean plants and tomatoes. Having a small oscillating fan gently blowing on seedlings (constantly) usually helps produce seedlings that can stand on their own. I often see these plants go to 3 or 3 1/2 times the height of their containers before needing a transplant.
Prepare some of your favorite soil mix in advance. A favorite, neutral (no nutrients) medium is equal parts of perlite, vermiculite, and sphagnum peat moss. This can be adjusted in many ways. For seedlings, I like to add a fist-full of worm castings to a gallon of this mix and add water with B1 to it.
If you are growing in hydro systems, you may want to use expanded clay pellets to fill your pots instead.
Root-bound plants are ready get transplanted into containers approximately 2-3 times the size of the ones they are currently in. There are two goals. One goal is to give the plants more room to grow. The second goal is to put the plants in a good position to colonize (grow roots through) the whole container very quickly.
When you transplant into a container that is too large, you end up watering a lot of soil that does not have any roots in it yet. It is very easy to over water this way. Try to avoid transplanting into containers that are excessively large.
When done, water the freshly transplanted plants with Thrive Alive B1, or another vitamin B1 product, to help reduce transplant shock. I generally turn the oscillating fan off the first day or two after transplanting young and tender seedlings... I also spritz them a little with plain water from a spray bottle. Older transplants do not need this kid-glove treatment and can be fed and watered as normal (but it don't hurt either).
In case you are wondering, I usually transplant clones and seedlings into plastic 16 ounce "party" drink cups. When ready, I move these into 6 inch or 8 inch containers (about 1 gallon containers). Finally, when the plants have outgrown these, I transplant one final time into 3 gallon containers (which are about 16-18 inches across). For me, the final transplant is into a flood-and-drain system, into containers of pre-soaked expanded clay pellets (which I use in all my hydro setups), in a room set for "flowering".