There's no way I'm aware of to change the size of the inputbox. You have a couple of choices:
1) Force line breaks with vbCRLF like this:
`message = "This line will end here: " & vbcrlf & "This appears on a new line"
2) Create your own form to act an input box. Depending on your VBA ability you can make a form with a textbox, label, OK button, and cancel button. Have that form use the string in the form opening arguments (OpenArgs) as the prompt text. That will give you a generic, customizable inputbox.