If there are a few rooms in your home that are frustratingly small in size, here are two tips that you could use to maximise the space in these areas.
Remove the wall radiators and fit floor heating in these rooms
If you have radiators attached to the walls of these compact rooms, you might want to get them removed and have floor heating installed instead. The reason for this is that most radiators are quite bulky and protrude out from the walls that they're attached to, which results in them consuming the scant amount of free space that is usually available in a small-sized room.
If you remove these and install a heating system below the floors, you can create additional space in these rooms (that you can leave unoccupied to create a greater sense of spaciousness or that you can utilise by putting some type of storage or furniture in the radiator's place) without compromising your ability to control how warm it gets in these particular parts of your home.
Whilst in a bigger room, you might need a combination of under-floor heating and wall radiators to fully warm up the space, the same is not true of smaller spaces, where you can usually quickly raise the temperature of the entire room using a single source of heat.
Readjust the position of any small shelves on the walls that are at eye level or below
You might also want to consider moving any shelves in your home's smaller rooms upwards if they are currently at eye level or lower so that they are much closer to the ceiling. There are two reasons why this could help you to achieve your space-maximising goals. Firstly, if these shelves are up by the ceiling, they won't use up space in the areas of the room in which you actually move around; this means that you won't bump into them all the time (which can happen when you're trying to walk across a particularly small room that is filled with a lot of stuff).
Secondly, it will allow you to retain this form of storage without you or anyone else in your home having to constantly see it when you're in these rooms, as most people do not spend much time looking at things above their own eye level. This means that the shelves and the items you put on them won't visually encroach upon any of the rooms' open spaces.