How do I change the size of a VHD?

There's a free utility you can download called VHD Resizer that should simplify this task.

There is also a step-by-step walkthrough of the entire process that might be of help.

You could use the /extend switch of Microsoft's VHD Tool.

VHD tool is an unmanaged code command-line tool which provides useful VHD manipulation functions including instant creation of large fixed-size VHDs. The source code is included.

Usage:

VhdTool.exe /create <FileName> <Size> [/quiet]
VhdTool.exe /convert <FileName> [/quiet]
VhdTool.exe /extend <FileName> <NewSize> [/quiet]

Create: Creates a new fixed format VHD of size <Size>.
        WARNING - this function is admin only and bypasses
        file system security.  The resulting VHD file will
        contain data which currently exists on the physical disk.

Convert: Converts an existing RAW disk image file to a fixed-format VHD.
         The existing file length, rounded up, will contain block data
         A VHD footer is appended to the current end of file.

Extend: Extends an existing fixed format VHD to a larger size <Size>.
         WARNING - this function is admin only and bypasses
         file system security.  The resulting VHD file will
         contain data which currently exists on the physical disk.