There is no specific parameter on height of stand vs size of modifier. What I mean by that is, you pick the support mechanism that will do the job based on where you want your light modifier to be placed. For example if I want my softbox to be on the ground I will use a floor stand base with a spigot or a baby plate and sand bag it. I might want the same modifier in a down light situation over a models head, then I would boom it with a super boom with a cine stand with counter weights and sand bags. In each of these scenarios I am using the same modifier but the support system is specific to the lighting position.
The best advise I can give you is buy the best quality stand your budget will permit, buy once, cry once. I still use my first stands purchases from 40+ years ago, Manfrotto Master Art. 004 stands and will likely live out my career with them.