64 = 2^6 = 4^3 = 6^2
It's a very nice power of two, and if you're a programmer then you'll know just how important powers of two are. Some examples (for valid ranges):
byte (VB: Byte): 0 to 255 (2^8-1)
unsigned short int: 0 to 65535 (2^16-1)
signed short int (VB: Integer): -32767 (-2^15+1) to 32768 (2^15)
unsigned long int: 0 to 4294967295 (2^32-1)
signed long int (VB: Long): -2147483647 (-2^31+1) to 2147483648 (2^31)
Floating-point numbers (e.g. float (VB: Single), double (VB: Double)) use a mantissa and exponent. The VB Decimal type is a scaled 64-bit integer, I believe (and I'm not sure what the valid range is). There are 92- and 128-bit data types, but they are actually implemented as structures.
To finish off:
1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, 4096, 8192, 16384, 32768, 65536, ... (don't know any higher ones without working them out).