TL;DR: Work at a mid-size startup, then decide to move up or down in the size spectrum.
It's a great place to learn a greater breadth of what you'd otherwise specialize in at larger companies. Once you've done that for some time, decide if you want to go deep or broad.
For continued breadth, work at more startups - maybe earlier stage, maybe later. Go with your interests.
For depth, work at companies where the problems you want to go deep on are just becoming relevant. For example, if you're interested in scaling X, work at companies that are just beginning to tackle those problems; ideally these companies are spinning up teams with this type of work.
It's a great place to learn a greater breadth of what you'd otherwise specialize in at larger companies. Once you've done that for some time, decide if you want to go deep or broad.
For continued breadth, work at more startups - maybe earlier stage, maybe later. Go with your interests.
For depth, work at companies where the problems you want to go deep on are just becoming relevant. For example, if you're interested in scaling X, work at companies that are just beginning to tackle those problems; ideally these companies are spinning up teams with this type of work.