It's important to understand the model that you are using when you consider changing marker sizes or locations - for example, the Helen Hayes model placed markers on short wand on the thigh and shank to define two planes (ankle-shank-knee and knee-thigh-ASIS) that allowed the HHH software model to measure limb rotations on each leg. Replacing the shank and thigh markers with markers on the skin surface without changing the model has significant effects on the limb rotation measurement accuracy.

In a more subtle way, changing the size of the markers placed directly on the skin surface will move the measured location (the marker centroid) - switching from 24mm markers to 5mm markers moves the measured location by 10mm. This may affect model calculations.

As far as measurement artifacts with markers on stalks - these can be reduced by careful mounting of the markers to minimize the motion - the original Helen Hayes wand markers used a long base, aligned with the limb, to provide better attachment and minimize vertical artifact motion. The software then processed the markers based on their location to reduce the motion artifact components.