Hi,
These is a api “vm_graphic_set_font_size()” in file " …\Arduino_IDE_for_RePhone\hardware\arduino\mtk\system\libmtk\include\vmgraphic_font.h "
can change font size. You can check it in file"…\Arduino_IDE_for_RePhone\hardware\arduino\mtk\libraries\LDisplay\utility\Display.cpp" on line 141 and 194.