how to send large strings (> 100 characters) from arduino serial monitor
I saw that arduino cannot read more than 63 characters at a time.


Not strictly true. The Arduino core HardwareSerial library has a 64 byte buffer, so it can only buffer up to 64 characters. If your sketch is reading the characters from the buffer whilst you are sending the characters from your PC then that limit is irrelevant. As long as you are reading the characters as fast as they are coming in.

Serial is actually quite slow (on the scale of things) and unless you're doing silly things like using delay() or long blocking loops, then reading the data fast enough is not that big a problem.  If you do need more time the simplest way is just to slow down the serial data somewhat - i.e., use a lower baud rate. The slower you send the data the longer you get between each character arrives to do things.

For strings the size you are on about you are probably best off processing the data on the fly rather than storing it all then post-processing it - simply because the Arduino doesn't have much RAM, so storing large strings isn't always an option.