Hi! Thanks for the easy question.
Yes. It's worth booting your OS from a solid state drive. It will effect things like latency when you click on folders and it will cause programs to load faster. Some people have described SSDs as the most profound change you can make to your computing experience.
The minimum size of an SSD depends on what you plan do you with it. If you're pro at internet surfing, watching videos, and reading e-mails then you can get 128GB. That would be completely adequate. If you're a gamer, then consider getting 256GB or more.
Don't get rid of your mechanical drive just yet. Keep it around for long term storage space.