There is a hardcoded limit of 100000 characters for output expressions, beyond which you will get this message. I should probably make it configurable or do something like what Mathematica does (allow for a sneak peak at the beginning of the expression), but I rarely have a need for this (if you encounter expressions like this, it is unlikely that you want to visually inspect them).
If you have an expression
ex which many terms and you just want to inspect individual ones, you can always do e.g.
to see the 213th term.