My question: Is there a limit to the number of characters which can be sent in a message?

For iMessages, no. For SMS the limit is 160 characters, but that has nothing to do with the phone; it's a hard limit imposed by the cellular network. However, the iPhone will allow you to send an SMS longer than 160 characters; it will break it up into multiple messages that observe the limit.

Lawrence Finch wrote:

However, the iPhone will allow you to send an SMS longer than 160 characters; it will break it up into multiple messages that observe the limit.

That's interesting. Thanks.