[LIP] Intel Opcode Query

Hi,
I am trying to write a disassembler for x86
processors. I have basic query on the format of the
instruction bytes.

Does every opcode that has "one memory operand" needs
to be "prefixed" with the "Operand-Size/Address-Size
Prefix" code?

Otherwise how do I decide which addressing mode to use
i.e. 16-bit or 32-bit?
For e.g.: here are opcodes and operands for ADD
instruction:

opcode      Instruction     Operand1        Operand2
0x00            ADD         r/m8            r8
0x01            ADD         r/m16           r16
0x01            ADD         r/m32           r32

r/m = bits-7,6,2,1,0 of MOD R/M byte
r/m8 = memory address for 8-bit data
r8 = 8-bit general purpose register ( eg: AL )

When opcode=0x00 how do I know which addressing mode
to use i.e. 16-bit/32-bit?

When opcode=0x01 how do I know which addressing mode
to use i.e. 16-bit/32-bit?
Also how do I know whether the registers are
16-bit/32-bit( eg: AX or EAX )?
So i need additional two pieces of information,
addressing mode and register size. Where do I get them
from?

I read the Intel Instruction Set Manual but didnt
understand how to go about. Any clue?

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