Is anyone aware of a limit on the amount of embedded code in SSRS? I have a report that contains 10-12 functions with a total of around 700 lines and when I try to type code in the Code window, nothing happens. If I try to paste more than about 700 lines, the code is truncated and some of it is lost.

Possibly this is a limit of the text box itself, not the .rdl file. You could try pasting a portion into the text box, save the file, and then paste the remainder directly into the RDL as you will then be easily able to find it.