Hi! Your sentence is very good. It says: Stars can be different sizes and colours.
This is grammatically correct. If you use American English, you can spell colours as colors: Stars can be different sizes and colors.
What the words mean (simple):
- Stars — things in the sky that shine.
- Can — means they are able to be that way.
- Different sizes — some stars are big, some are small.
- Colours / Colors — stars can look yellow, red, blue, or white.
So your sentence is correct. Great job!