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Extend sight lines as far as possible and try to eliminate anything that interrupts movement of the eye from corner to corner.
Clear-glass shower enclosures or sheer shower curtains will contribute to a feeling of open space in a small bath.
A floating vanity (mounted on the wall rather than anchored to the floor) or a pedestal sink opens up the floor area and can make a room feel larger.
When practical, use half-walls instead of full partitions. Or, use materials like glass blocks or frosted glass that won't cut out light.
Raise the ceiling or install a skylight.
Use a pale, uniform color scheme. Tone-on-tone is another useful trick to expand space.
Keep window treatments unobtrusive.
Light the space well to eliminate shadowy areas.
Install floor and wall tiles on the diagonal to extend the visual length of the space.
Keep accessories simple to avoid a too-cluttered look.
Use mirrors to bring in light, reflection, and sparkle to your space.