Remember you will want to download and install the Windows Virtual PC first.
Windows Virtual PC: Home Page
The first step and the one most advised before downloading that and the XP Mode file in the 32bit or 64bit flavor which would be you there running the 64bit 7 Pro would be the "Microsoft® Hardware-Assisted Virtualization Detection Tool".
Download details: HAV Detection Tool
With Intel some of their cpu models won't support it. We'rve already gone over quite a bit of this in the guide there. With everything set once you have gone over that and have the VPC on which generally requires a full system restart you can then run the XP Mode installer. All this is covered in the guide there.
As far as sizes the 64bit Virtual PC is roughly 9.87mb while the XP Mode itself is quite a bit larger at 479mb for the 64bit flavor. It's almost the size of XP Pro on an original full install disk sizewise but runs on the VPC.