Ah ok, now makes more sense... Of course compared with default wm's (or whole desktop environment) in some other distros, busybox could be considered relatively small, it is all relative
You can use "manager" as a search term in appsbrowser, to have them filtered out
Looks like tinywm being smallest, also evilwm being pretty small, but question is also what is minimal functionality you request for your purposes.
For reference: my jwm-snapshot is using 2760 Kb of RES memory atm.
EDIT: checked flwm_topside as in base of tinycore: 1948 Kb RES memory
But isn't there a more direct way to accomplish this by disabling the dynamic menu in the first place?
I'd guess that might also depend upon configuration of particular wm.
In jwm i think you could just try to comment the line
<Include>/usr/local/tce.menu/menu</Include>
in ~/.jwmrc out, or possibly replace it by an 'include' pointing to your own customized menu file.