Choose a position that uses existing vertical space while preserving clear routes across the room. Mark the complete footprint, including any projecting platforms, and walk around it before ordering.
A corner beside a seating area may work better than a narrow gap that interrupts access to a cupboard or radiator. Visual crowding also depends on what surrounds the tree. Removing an unnecessary side table or simplifying nearby ornaments can create space without sacrificing the cat's resting area. Do not squeeze the structure behind furniture if that blocks entry, cleaning or required fixings. Remember that the cat needs room to get on and off, not merely enough space for the base to stand. A slightly shorter tree with generous platforms may suit a particular room more comfortably than the tallest model that technically fits.
Lay out the footprint with removable tape, then temporarily place a box within it. This makes the occupied space easier to notice than a flat outline alone, but still does not represent upper platforms or their overhangs.
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