What does error in redirect data: included file xxxx is too big mean?

You are trying to include a very large file in a redirection list, using the :include: feature. Exim has a built-in limit on the size, as a safety precaution. The default is 1 megabyte. If you want to increase this, you have to rebuild Exim. In your Local/Makefile, put

MAX_INCLUDE_SIZE = whatever

and then rebuild Exim. The value is a number of bytes, but you can give it as a parenthesized arithmetic expression such as (3*1024*1024). However, an included file of more than a megabyte is likely to be quite inefficient. How many addresses does yours contain? You get the best performance out of Exim if you arrange to send mailing list messages with no more than about 100 recipients (in order to get parallelism in the routing).