Making things fit in your inventory can prove to be quite a hassle sometimes. In real life objects don't have a default orientation and it feels unnatural to be restricted by this in DayZ.
Allow rotation of items. This of course would only be necessary for objects that take an unequal amount of horizontal/vertical slots. Rotating could be done by selecting the option from the right-click menu, but another idea would be to rotate upon right-click when the item is being dragged. This would be quite a natural way of interacting: You grab something, rotate it, and put it back in your backpack/pants etc... This is also the way that would require the least amount of actions by the player.