The previous person complained that this is all retro, and not a good retro. Perhaps. However, from the viewpoint of wanting to use as a base for both chest nodes and inventory item nodes, it being small is an advantage. As for the images, sure they kind of clash with minetest asthetic, but really, again, excellent starting point. If you don't like the pictures, give it different ones.
Things to note: The folder is best with tools, since you can only take one item away at a time.
Also, you tried changing the inventory size, without changing the GUI size the same way. Therefore, the inventory probably has been changed, but the user can't tell, not without testing. So, if you want to change the size of the inventory, this is everything you need to change (#, being the numbers specifically, I recommend you changing, when you want to change).
"size [#,#] --I recommend this to be [The larger of inventorysize first # or 8, inventorysize second # +5]
"list[nodemeta:" .. spos .. ";main;0,0.3;#,#;] --I recommend this to be [inventorysize first #, inventorysize second #]
"list[current_player;main;0,#;8,1;]" .. --I recommend this to be [inventorysize second # +.85.] This one specifically, is the hotbar.
"list[current_player;main;0,#;8,3;8]" .. --I recommend this to be [inventorysize second # +2.08] - This is the top of non-hotbar inventory
default.get_hotbar_bg(0,#)) --I recommend this to be [inventorysize second # +.85.] --Hotbar also.
inv:set_size("main", #*#) --And of course, the dimensions of the slots you actually want it to have.
Also note, that it generates the inventory slots of the object on place, which means, if you want to check to see that the GUI matches what it should have as inventory slots, you will need to place it down after each check, as the ones already placed down will not be refreshed until they are picked up and placed.