Since the whole structure is of variable size, one way to do this is like this (I can't test it because I don't have this dll on my system):

string volume = "";

int size = 0;
// ask for whole structure size
FbwfFindFirst(volume, IntPtr.Zero, ref size); // this call should return ERROR_MORE_DATA which is ok

// allocate for the structure
var ptr = Marshal.AllocHGlobal(size);
try
{
    FbwfFindFirst(volume, ptr, ref size); // should not return error

    // get the easy part 
    var detail = Marshal.PtrToStructure<FbwfCacheDetail>(ptr);

    // compute filename offset and get the string
    // file name length is in bytes, per documentation
    var fileName = Marshal.PtrToStringUni(ptr + Marshal.OffsetOf<FbwfCacheDetail>("fileName").ToInt32(), detail.fileNameLength / 2);
}
finally
{
    Marshal.FreeHGlobal(ptr);
}

[DllImport("fbwflib", CharSet = CharSet.Unicode)]
static extern int FbwfFindFirst(string volume, IntPtr cacheDetail, ref int size);

[StructLayout(LayoutKind.Sequential)]
struct FbwfCacheDetail
{
    public int cacheSize;
    public int openHandleCount;
    public int fileNameLength;
    byte fileName; // don't use this member
}