What's a proper window size in RSA exponentiation?
Window Size (W) for many practical implementation of RSA Algorithm (like OpenSSL) is related to key length.for example in openSSL library for insecure 80 bits key, W is 4 or for insecure 320 bits key , W is 5 and for 1024 or 2048 bits key (length), W is 6.

note:Maximum of Window size in openSSL Library is "6"