If you’re growing photoperiod plants, it makes sense to keep them in the “right size” pot. This will make watering and the feeding process much simpler. The right size pot for you will depend on what you have available and what size plants you’re trying to grow.
When transplanting, a general rule of thumb is to double the size of the pot.
I start in 4” pots - solo cup size. After that, I have some 1.5 gallon “training” pots where I’ve drilled holes along the rim for tying branches down. I’ve also some got 3 gallon plastic pots I’ve given the same treatment to. Different sizes may work better for you.
Before you consider transplanting, the plant should have filled the current pot. A couple of signs where it might be ready is when the leaf canopy is extending beyond the edges of the pot (the best measure), or if the soil in the pot is pulling away from the sides between waterings.
Before I start yanking plants from their current homes, I like to have everything prepared. The plant has been watered in well, a few hours prior. I have a bin of my soil mix ready, new pots lined up, mycorrhizae - everything I need.
I get an estimate of the amount of soil to add to the bottom of the new pot by placing the old pot + plant inside the new one. I like to use the bin as a temporary landing spot for the root ball; so I like to smooth out the soil in there first.
I like to turn the pot 90 degrees and let any loose dry soil fall back in to the bin. I try to keep it from hitting the leaves but you’ll have some that inevitably does. That’s why I like to save any leaf pruning I’ve got planned until after.
While supporting the stem and root ball, tip the pot a little more and try to dump it into your hand and slide the old pot off. Don’t yank on the stem - squeeze and work the old pot a little if it won’t go. Sometimes if it’s been in the old pot too long it gets stuck. Give it a shake if you need to.
Once it’s out carefully set it in the bin in the prepared spot. Apply mycorrhizae if using and transfer the plant to the new container. Place the new container in the bin and start filling the new pot.
Once it about halfway full, I like to “seat” the plant. You can still move it a little at this point. Press down on the root ball and adjust as needed.
When I’m filling the new pot I don’t press down to compact the soil - I want it loose. With plastic pots, I like to give them a little controlled drop to settle everything - I sort of tap it on the floor a couple times, then add more soil as needed. With fabric pots, I shake the sides repeatedly as I’m filling.