Perfect size for starting seeds in soil is the standard size plastic solo/dixie cups...make sure they are the ones with the white inside and red/blue/green whatever solid color outside..never clear of course. Go for the good ones even if they cost a few bucks extra it's worth it. They are easy to write on with a marker to keep track of what's what or even just scratch words into with a nail/screw if you can't find that marker that is always missing lol. For drainage just poke or drill a few holes in the bottom before planting.

The deep tapered shape/size of the cup is perfect for establishing strong rooted plants ready to go into 3 gallon or bigger containers and I know of someone who once had full grown 2 ft tall flowering plants in regular solo cups with good nutrition and quality soil (with coco fiber & polymer crystals added) and obviously lot's of attention to watering...pain in the butt to water alot but very possible if you are attentive. Plant's do not need as big of a container as you might think and sometimes will actually flower faster & better when they have less new root space (provided they are well established already) or else it will be doing too much new vegetative and new root growth instead of putting all of it's energy into reproducing in the form of it's flowers come happy time.

Simple test to prove this is flower two identical well rooted clones under the same light at the same time with exact same fertilizer regimen and transplant one into a 3 gallon bucket a few days before flowering and the other one leave in the solo cup nearly root bound...the one with less root space to grow will flower much faster and fuller with a higher yield before the larger container plant because it is putting all it's energy into flowering while the other one does more growing under the soil and diverts energy from budding.

In real world terms this means do not transplant right before flowering, at least 10 days prior minimum & the longer the better..always make sure plants are very well & fully rooted in their containers before doing so and you will end up with a much faster & fuller crop.