Answer
The usual limit is 1,024 pixels on the longest side. If a local check detects lettering, the generated dimensions are preserved up to 2,048 pixels. An uncertain or unavailable check also uses the 2,048-pixel limit to preserve detail. Smaller images are never enlarged. This processing adds no extra charge and can appear as Preparing image.
Text detection can miss lettering and does not correct unreadable or inaccurate text. Existing saved images and composer upload sizing stay unchanged. These rules apply to private and People chats, including scheduled Image requests, on web, iOS and Android.