Very simple mod to decrease stack sizes with the following "formula":

if item.stack_size > 200 then
    item.stack_size = 100
elseif item.stack_size > 100 then
    item.stack_size = 50
elseif item.stack_size >= 50 then
    item.stack_size = 25
elseif item.stack_size >= 25 then
    item.stack_size = 10
elseif item.stack_size >= 10 then
    item.stack_size = 5
elseif item.stack_size >= 5 then
    item.stack_size = 2
else 
    item.stack_size = 1
end