When choosing a new pot for rosemary, look at the existing root ball before looking at the size of the balcony. A generous decorative container can surround a small plant with more wet growing medium than expected.
Check drainage and the condition of the current mix. If the plant is struggling because the medium stays wet, increasing the volume without changing its behaviour may not help.
Repot for a reason
Congested roots, unsuitable medium or a damaged container are specific reasons to review the arrangement. A plant merely looking small beside its neighbours is a display issue.
Use a potting medium appropriate to rosemary and keep the drainage outlet clear. After repotting, check the root area before watering instead of keeping the previous timetable unchanged.
Keep a note of the container size, mix and date. If the plant later slows down, you can compare its conditions before and after the change. A new pot is part of a growing system; it does not solve weak light or an unsuitable watering routine on its own.
The RHS container guide advises using the root ball to choose a pot and warns that surplus compost in an oversized container can become waterlogged. For rosemary, that makes root size and drainage more useful criteria than decorative impact alone.
Related reading: Potted Rosemary: Choosing a Container and Potting Mix.