How to code large forms?

without seeing the code I can't really comment.
I wouldn't worry about long forms.

If your just worried about the length of the box you could use some Javascript/Ajax to create a single form that has steps to progress through. 

You basically have a set of layers that are intially invisible in the form, each layer has a set of form controls on it and a next button. u click the next button and the current layer hides and reveals the next. on the last layer you have the submit button.