Caster wheel are usually located at the rear of the wheelchair.
Wheelchair: Caster Load
Caster load (the weight on the casters) is a factor that affects the ability of the casters to roll over obstacles and the lower the caster load the smaller the casters can be and still function well. Caster load has a greater effect if the casters are located on the front of the wheelchair regardless of whether the wheelchair is electric or manual. Caster loading is a factor that experienced wheelchair technicians take into account when setting up a wheelchair for a user, if possible, try to find an acceptable load level for function.
Wheelchair: Caster Diameter
The diameter of wheelchair Casters wheels for extreme rough terrain wheelchairs with the most common diameters being 6" and 8".
Generally the larger the caster diameter the easier the wheelchair will be propelled over obstacles and rough ground. On the other hand, the smaller the caster diameter the easier the caster will swivel and the more responsive the wheelchair will be when turning.
Based on this, one would think that the little 6" casters would be almost impossible to use outdoors however because the type of wheelchairs that use the tiny casters have their center of gravity centered on the rear wheels and there is very little weight on the casters.
Wheelchair: Caster Width
Caster width is another factor that affects how a caster will roll over rough terrain. The wider the caster tire, the easier rough terrain can be travel-led over. Most manual wheelchair casters are around ¾" wide with 2" being considered wide and power wheelchair casters can be between 6" and 8" wide.