When you type from the subject line, automatically move to the text box when you run out of characters
I'd prefer a box with a countdown of the number of characters remaining (or count up relative to the total number available) to still give guidance on when you're running out of space, but avoids problems where the cursor jumps around unexpectedly. This is especially a problem when attempting to re-enter the textbox after it has been filled to edit what was previously typed.
Daniel Meservy commented
you could have it break at the space character in the string...