fread() already allows specifying the number of bytes. For serial object, the input buffer size is a double value: https://www.mathworks.com/help/matlab/ref/serial-properties.html#mw_5d33f1bd-97a2-4474-b2ea-8499775ddce4 it needs to be constant. You can try using BytesAvailableFcn: https://www.mathworks.com/help/matlab/matlab_external/bytesavailablefcn.html callback function and load the data from the serial buffer to the base workspace (for example, using a global variable) every time the number of bytes in the serial buffer reaches a specified number.
However, starting from R2019b, MATLAB has introduced a function serialport: https://www.mathworks.com/help/matlab/ref/serialport.html which does not require to specify the size of the input buffer and handle it in the background.
