Indoor Stem height vs root size

[Hybrid]

I know I shouln’t repot autoflowers to often so I plan to use a relative large pot for the seedling, deep enough for the roots to grow free downwards until first repotting 3-4 weeks old(when they start flowering it forcing me to bring them outside and then I repot to the final size or put it directly to the grown). So what potsize do I need as a first pot to avoid the roots to hit the bottom of the pot until flowering? Can the plant height tell how deep the roots have gone? Have anyone used transparent plastic pot to see how deep and wide they dig?