Would be great to be able to set max. storage of the different items.

The overall maximum storage-capacity of the building itself. Not of each item.
but might be a idea to alter the size of Items.

Does this allow me to choose max capacity for each ITEM in storage or just the storage as a whole in a specific barn?

yes there are already 4 mods but this one is the only one updated so ....
THX Lumpous

This mod was intended for a friend of mine who doesn't wanted to subscribe a dozen mods...

But technically you are correct - if you are willing to install 4 different mods to achieve the same you could change with mine.

Useless, there are already 4 Mods or more that do the exact same