Honest Optical RecognitionFive Factors That Decide Image to PowerPoint OCR Output
None of these is a threshold you can look up, because the engine weighs them together.
Pixel height of the lettering, and contrast against the background. Characters need enough pixels to carry their shape, and body copy you already struggle to read at full size will come back as a picture; dark grey on mid grey, a watermark behind a paragraph or a photograph showing through all reduce the separation the engine needs.
Straightness, and whether the writing is printed. A scan sitting a few degrees off curves the baselines, and the engine reads in lines; marker pen, biro and signatures are not what a recogniser trained on typeset characters learned.
And which script it is in. The installed packs cover English and Simplified Chinese, which between them supply the Latin and Han character shapes. Scripts built from neither, Japanese kana, Korean hangul, Cyrillic and Arabic among them, normally stay as picture.