Begin with your cat's habits and the room's working layout. Identify where the cat already rests, watches activity or scratches, then consider furniture that offers those opportunities without obstructing people.
Mark the full assembled footprint on the floor, allowing for projecting beds and space to approach the lower level. Check doors, curtains, drawers and nearby seats in their normal positions. Once suitable sizes are clear, compare finishes and fabrics with the room. A tree that matches the sofa but occupies a busy walkway is unlikely to be the best choice. Likewise, a discreet corner may not appeal if it is completely separated from your cat's preferred part of the home. Use appearance as one part of the decision, alongside load information, usable platform dimensions, cleaning access and the installation requirements.
Test the proposed position during an ordinary evening: open the door, pull out dining chairs and sit in the usual seats. A location that works in an empty room may interrupt the routes people use once furniture is in use.
For product comparisons alongside this advice, see Linden wooden cat tower and Arbor Retreat. The details below explain their sizes and uses.
Further reading: cats.org.uk.
Linden wooden cat tower
Arbor Retreat
Floral Canopy