Allow users to override the hardcoded repr maxsize value

This change should enable users to increase the size of the error
messages without being forced to increase the verbosity level, which
has lost of side-effects.

changelog/10589.feature.rst

@@ -0,0 +1,2 @@
+Allow users to override repr maxsize value (240) by defining
+:envvar:`PYTEST_DEFAULT_REPR_MAX_SIZE` environment variable.

doc/en/reference/reference.rst

@@ -1061,6 +1061,11 @@ processes can inspect it, see :ref:`pytest current test env` for more informatio
 
 When set, pytest will print tracing and debug information.
 
+.. envvar:: PYTEST_DEFAULT_REPR_MAX_SIZE
+
+Setting this would override the implicit 240 chars limit used for truncating
+longer error messages.
+
 .. envvar:: PYTEST_DISABLE_PLUGIN_AUTOLOAD
 
 When set, disables plugin auto-loading through setuptools entrypoints. Only explicitly specified plugins will be

src/_pytest/_io/saferepr.py

@@ -1,3 +1,4 @@
+import os
 import pprint
 import reprlib
 from typing import Any
@@ -96,7 +97,7 @@ def safeformat(obj: object) -> str:
 
 
 # Maximum size of overall repr of objects to display during assertion errors.
-DEFAULT_REPR_MAX_SIZE = 240
+DEFAULT_REPR_MAX_SIZE = int(os.environ.get("PYTEST_DEFAULT_REPR_MAX_SIZE", 240))
 
 
 def saferepr(