local INV_WIDTH = 8
local INV_HEIGHT = 6

minetest.register_on_joinplayer(
    function( player )
        local inv = minetest.get_inventory( { type = "player", name = player:get_player_name() } )

        local list_name = "main"
        inv:set_size( list_name, INV_WIDTH * INV_HEIGHT )
        inv:set_width( list_name, INV_WIDTH )

        local s = player:get_inventory_formspec()
        -- change view size
        s = s:gsub( "(size%[)([%d%.]+),([%d%.]+)(%])",
            function( prefix, w, h, suffix )
               return prefix .. INV_WIDTH .. "," .. ( INV_HEIGHT + 2.5 ) .. suffix
            end
        )

        -- change number of slots
        s = s:gsub( "(list%[current_player;" .. list_name .. ";)([%d%.]+),([%d%.]+);([%d%.]+),([%d%.]+)",
                function( prefix, x, y, w, h )
                    return prefix ..
                           x .. "," .. y .. ";" ..
                           INV_WIDTH .. "," .. INV_HEIGHT
                end
        )
        player:set_inventory_formspec( s )
    end
)