The size of a pilot study for a clinical trial should be calculated in relation to considerations of precision and efficiency.

By J. Sim, Martyn Lewis. Journal of Clinical Epidemiology, 2012

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Sample size requirements to estimate key design parameters from external pilot randomised controlled trials: a simulation study (2014, methodology): “A frequently reported problem with publicly funded RCTs is that the recruitment of participants is often slower or more difficult than expected, with many trials failing to reach their planned sample size within the originally envisaged trial timescale and trial-funding envelope.”
An audit of sample sizes for pilot and feasibility trials being undertaken in the United Kingdom registered in the United Kingdom Clinical Research Network database (2013, background): “Continuous 6 43 (15, 60) [10, 60] However, Arain et al.”