Is it a good idea to introduce the main characters before giving the short synopsis in a query letter?
For example: “Sara, is nine feet tall, terrified of spiders and in love with a bald teenager etc…….”
If this is a bad idea, what is the best way to introduce main characters in a 250 word query?
This is a very bad idea. You are not required to introduce main characters at all, unless they are part of your novel description. In fact, listing characters and telling us unnecessary information about them is a terrible idea and a real turn-of. Don't do it. The characters mentioned in your introduction are enough.
The tendency to put character's names in bold is based on the old screenplay-writing standard where when a new character appears, their name is all in caps. This does not apply to prose.