Edit /usr/share/gnome-shell/js/ui/dash.js
Change
...
const Dash = new Lang.Class({
Name: 'Dash',
_init : function() {
this._maxHeight = -1;
this.iconSize = 64;
this._shownInitially = false;
...
to
...
const Dash = new Lang.Class({
Name: 'Dash',
_init : function() {
this._maxHeight = -1;
this.iconSize = 32;
this._shownInitially = false;
...
and further down the script
...
let iconSizes = [ 16, 22, 24, 32, 48, 64 ];
...
to
...
let iconSizes = [ 16, 22, 24, 32 ];
...
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