I am building a web app that requires user input of text and images to be placed on a PDF page which will later be printed as a letter. This page will have a specified area for text and a specified font. This font will be a variable width custom font, but the user will not be able to change the size or apply any formatting to the text.
My solution so far is to set up a text area input of a fixed width and height, then using javascript, detect when a user has typed more text than will fit without scrolling and stop their text entry and display an alert. Since I can use custom fonts in that text area I should be able to get a really close approximation of the limited space available on the letter and prevent the user from adding too much text.
To make matters more complicated, the letter will have a front and back page with equally sized text areas, and I need to show that the text will flow to the back page.
Here's the edit screen as it's working right now. Can you please give me some suggestions about how to make this more user friendly? Will users understand this when they see it? We will be doing formal user testing in a few weeks probably.
Users will be able to preview the letter with all the images and the text in place on the page after they are done writing.
Update: Here's a new mockup taking into account some of the suggestions below.