Unlike flat screen interfaces, interactive elements are placed at different distances from the agency or viewer and user can interact with them either by pointing controller ray or gaze depending on the type of VR hardware.
Fat Finger problem
Referred to as the “fat finger” problem in traditional screen medium UI, it makes UI unusable. MIT Touch Lab study found that averages for finger pads are between 10–14mm and fingertips are 8–10mm. This makes 10mm x 10mm a good minimum touch target size. So irrespective of the size of the fingertip of the user, this size is good enough to accommodate the area of interaction.
The laser pointer is the new mouse cursor
The pointer in VR UI has the same size irrespective of the hardware and user hand size. Because the interactions are always handled by the controllers that cast a laser ray that travel far off to intersect with different elements.
As the distance between the viewer and the interactive elements increases the chances of missing to select the target by pointing increases. And when it’s near it should resemble the size of touch screen based interactive targets that are at arms distance. This is synonyms to an interactive touch screen unit.
Not just size but the distance also
Interactive elements must be at least 50cm × 50cm (19.6in × 19.6in) placed within the distance range of 2 to 4 m, to support adequate selection time and prevent fat-finger errors.
In order to achieve maximum comfort in interacting with elements, they should be not only be sized properly but placed or made active within a given distance from the viewer.
Reticle vs Pointer
One distinction that needs to be seen is that between the size of interactive elements for reticle and for the pointer.
With reticle, users interact with interactive elements using gaze which is controlled by head movement, hence very limited ergonomic movement.
With a pointer, users interact with elements using laser pointer which is controlled by hand and wrist movements, giving much more flexibility in terms of ergonomic movement.
In either case, we need to understand that as the distance between the user and the element increases the chances of missing the target also increases if the size of the element remains the same irrespective of distance.