Allow override of DEFAULT_REPR_MAX_SIZE

Fixes: #6682

.gitignore

@@ -58,3 +58,6 @@ pip-wheel-metadata/
 
 # pytest debug logs generated via --debug
 pytestdebug.log
+
+# other
+.DS_Store

src/_pytest/_io/saferepr.py

@@ -1,3 +1,4 @@
+import os
 import pprint
 import reprlib
 from typing import Optional
@@ -93,7 +94,7 @@ 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(

src/_pytest/helpconfig.py

@@ -217,6 +217,8 @@ def showhelp(config: Config) -> None:
         ("PYTEST_PLUGINS", "Comma-separated plugins to load during startup"),
         ("PYTEST_DISABLE_PLUGIN_AUTOLOAD", "Set to disable plugin auto-loading"),
         ("PYTEST_DEBUG", "Set to enable debug tracing of pytest's internals"),
+        ("PYTEST_DEBUG", "Set to enable debug tracing of pytest's internals"),
+        ("PYTEST_DEFAULT_REPR_MAX_SIZE", "Set custom size for using ellipsis to truncate long strings."),
     ]
     for name, help in vars:
         tw.line(f"  {name:<24} {help}")