SlickEdit has all kinds of adjustable settings. Too many to put in Tools|Options, it would be confusing and overwhelming. The various "def_blahblah" kind of settings are adjusted via the "set-var" command at the SlickEdit command line. It would be nice for the error message to at least point at the "set-var" command, it seems to assume a certain level of familiarity with SE that may not be true in all cases.
In CUA and most emulations press Escape to get to the SlickEdit command line. Then type
set-var def_update_context_max_file_size (it will show autocompletions for you as you type), then press Enter, then adjust the value.
It is defined in the macros\context.e file (you can use "f
symbolname" at the SlickEdit command line to find symbols), and initialized to a default value of 4MB.
The warning is letting us users know that the context features will not be available in that file. It's probably good there is a warning, otherwise the product would seem to mysteriously not work, and we'd be complaining about the lack of a warning.

Increasing the value of def_update_context_max_file_size could make it take longer for SlickEdit to open large files -- so you might prefer to increase the value.