Is it possible to disable TTF manual hinting instructions above a given body size?
Two ways:
You can define until when to suppress overshoots with the TTFOvershootSuppressionBelowPPM parameter in Font Info > Font.
And in theory, you could set the rendering mode for different PPMs with the GASP table, which can be set with the GASP table parameter, also in Font Info > Font. Theoretically a GASP table could also specify ‘from PPM x to infinity, do not use gridfitting’, but that would at least be pretty unusual (IIRC Microsoft recommends a specific set-up for GASP, and this is how it is implemented in the parameter UI). Two problems: Not every renderer respects the GASP settings; and the implementation in Glyphs is currently somewhat inflexible, you can only tell it below which size gridfitting is supposed to be off.
Why do you need this?