I found from your update page (http://xdebug.org/updates.php)
"Allow xdebug.var_display_max_data and xdebug.var_display_max_depth settings of -1 which will unlimit those settings."
Which is cleaner than use a high value like 999.

However, for these two settings, you did not document it in your all_setting page:
http://xdebug.org/docs/all_settings#var_display_max_data
http://xdebug.org/docs/all_settings#var_display_max_depth

You should update documentation.

On a related note and that should be directed to the person who take care of the software design:
Semantically "-1" is also not obvious, unless you know it. I know developers sometime use -1 or 0 as unlimited (to keep the same data type for their setting), it can be OK, but I often try to be "right" on the value I choose. The real answer is that there is no maximum. We can therefore use a value like null to express there is no maximum (or we can think and create/support a constant/alias value for settings like */undefined/unlimited/none/null/etc.). If you choose to go that route and want feedback, I will be glad to put my 2 cents about what I think is the right value/name/etc.