I believe the 16in Apple Mon is a multi-sync monitor, you would need either the 13in color, the 12in color (sold with LCs) or the 12in B/W/Grey monitor (also sold with LCs) to work with the IIsi.

Or a multi-sync monitor with VGA capability (I use the Gateway EV700) with a Mac Adapter with switches, and set the switches at 640X480 or 400 - LC Monitor.

Q: Does the IIsi has VRAM in it? Wait... The Macintosh IIsi shares system memory for video function: VRAM: 64-320 k (shared with system RAM), so that can't be it.