There isn’t much standardization, but I can say how I define them, which is based on the places I have worked. The years are just a guidepost, notice they overlap as it all depends on the individual.
Jr Designer (or designer level 1 in some orgs): novice, 0-3 years experience. Needs lots of guidance and direction. Brings lots of fresh ideas, new approaches, energy, enthusiasm, and is a sponge.
Designer (or designer level 2 in some orgs): journeyman, 2-6 years experience. More independent. Self starter. Needs feedback, but not as much hands on guidance. Can present to and communicate with other departments (engineerings, marketing, product line management). Has an decent level of understanding of costing and production techniques. Can get things done on time.
Sr Designer: expert, 5-10 years experience. Completely independent operator. Needs very little feedback. Can run some programs completely, can present to execs, can mentor younger designers. Is a role model to younger designers. Understands the product development process and can set own time tables, and has a firm understanding of costing, and production techniques.
Principle: uber expert, 8-20 years. A true expert practitioner. Basically a Sr, but just better. Could have been a director, but just didn’t want to be. Not all places have the budget to afford this.
If I had a 10 person team I would look to have a mix of:
5 Jrs
3 level 2s
2 SRs or Principle