Matlab to Arduino communication
You cannot send larger values because byte only covers the range from 0-255. 

To send larger values, you can break your int variable into 2 byte variables. Here's an example:

// On Arduino
int myVar = -123;
byte myVar_HighByte = myVar&gt;&gt;8; // get the high byte
byte myVar_LowByte = myVar; // get the low byte
// x86 compatible machines are little-endian so we send the low byte first
Serial.write(myVar_LowByte); 
Serial.write(myVar_HighByte);

% On MATLAB
s = serial('COM10')
fopen(s)
myVar = fread(s,1,'int16')


Disclaimer: Syntax might not be entirely correct, since I have no MATLAB or Arduino near me when I typed this. But you should get the idea. ;-)

Edit: On second thought, it might be easier to use a pointer. 

// On Arduino
float myFloat = 3.14159265359;
byte* ptr = (byte*) (&amp;myFloat);
Serial.write(*ptr++);
Serial.write(*ptr++);
Serial.write(*ptr++);
Serial.write(*ptr);

% On MATLAB
s = serial('COM10')
fopen(s)
myVar = fread(s,1,'float')


Having said that, you will still have to take care of the endianness, if you use a different microcontroller.