You can use a file gtkrc, make it in your Gimp profile to change some sizes.
As ever with Gimp there is a plugin
Change menu & icons.py to make that file for you.
see: menus and dropdowns too small - Page 1 • GIMP Chat
this a before after screenshot(s)
That plugin is python and some linux no longer has the gimp-python package so the gtkrc file might look like this.
gtk-theme-name = “Default”
style “user-font”
{
font_name=“Sans 14”
}
widget_class “*” style “user-font”
gtk-icon-sizes = “gtk-menu=30,30”
edit: Sorry that does not really answer your question. The toolbox icons resize is via the Gimp preferences and there are already requests to make the maximum size larger for HD displays. That might never happen with Gimp 2.10
Until a fix, there is always the Tools drop down menu in conjunction with that gtkrc file
