I'm using 1.3.7 and it looks like [command] doesn't work in menu. man fluxbox-menu doesn't make it clear how to use it, but I've tried different ways and neither works.
I suppose it should be possible to do things like:
[command] (Description) {MacroCmd {ForEach {ResizeTo 1604 1026} {Matches (Class=Chrom.*)}} …} <icon.png></icon.png>
Try [MacroCmd {ForEach {ResizeTo 1604 1026} {Matches (Class=Chrom.*)}} …}] (Description) <icon.png></icon.png>