String size

I need to know how many bit of data a String can hold in arduino.

Previously in my project I used to send 240 bit (30 byte) of data through arduino serial port and I will receive it in a string variable through Serial.readString() function.

Now in my new project I need to send 30 byte of data through serial port. Is it possible to receive it in a string or any alternate procedure is there to receive data.

The data I will be sending is 0,1 only, its a binary form of an image.

I am using arduino mega for my project

Previously in my project I used to send 240 bit (30 byte) of data

That does NOT sound like something that would then be stored in a string or a String.

The data I will be sending is 0,1 only, its a binary form of an image.

Why are you converting the binary data to a string?

Take a look at Serial input basics - updated for ideas on how you might do it if you really need to.