it can depend on how old your computer is but it is still worth it with windows 7. 250GB is the sweet spot when it comes to buying an SSD drive. and i found windows 7 boots up a touch quicker then windows 10 on an SSD
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.
Agreed with the other folks above...putting the OS on an SSD is an easy Yes answer.
In my system, I have a 240GB SSD specifically for my copy of Win7 and any associated drivers/programs/etc. I also have a 960GB SSD for more modern games (faster load times for maps and round loading on World of Warships for a specific example, among others) and my older 1TB HDD for older games and music that don't need the speed of an SSD specifically. That way I don't worry about space issues on any of the drives.