Some window managers will only support certain icon sizes. These window managers store an attribute
property('WM_ICON_SIZE', wm_icon_size(List))
on the screen's root window. You may use
get_window_attribute to find out these icon sizes. List
will be a list of terms of the form
size_range(Min_w,Min_h,Max_w,Max_h,W_inc,H_inc)
where Min_w and Min_h specify a minimum icon width and height, Max_w and Max_h specify a maximum icon width and height, and W_inc and H_inc specify a width and height increment. For example, if List were
[size_range(64,64,64,64,0,0), size_range(32,32,48,48,8,8)]
this would mean that legal sizes are 32 by 32, 40 by 40, 48 by 48, and 64 by 64.
You should not need to set this attribute, though you could if you wanted to.